In games where you can learn spells or techniques, what's your favorite method of learning them?

For purposes of this topic, a technique is anything other than a spell that is actively used in a manner similar to a spell. This could, for example, include various weapon techniques, like the ability to attack twice or to sacrifice accuracy for high damage. One requirements is that these have to be active abilities; that is, using them must require some action on part of the player (like choosing it in a menu or pressing the hotkey assigned to it) rather than being passive abilities.

Some of the methods that I've seen for learning spells and techniques:
* Learn them automatically at a certain level
* Learn them automatically at a certain job level (for games with job systems)
* Purchase them at a store
* Find them somewhere in the world
* Spend a currency gained at level up (like skill points or Wizardry 6-8 style spell picks; D&D 3e Sorcerers would also count as this)
* Spend a currency, other than money, that's independent of level (see Final Fantasy Tactics)
* Earn points that are independent of XP (Chrono Trigger)
* Equip an item, and you can now use the ability as long as it's equipped
* Raise skill to specific value (Dragon Quest 8/9)
* Sparking: When you use a technique, you have a chance to learn another of the same type
* Randomly learned after battle
* Randomly learned on level up

(And there are certainly others not on this list)

So, what's your favorite method of learning spells and techniques?

In this case I tend to favor the realism brought by needing trainers for this. Some may just help you if you ask, many for money, some have quests to solve or other conditions before they will, but rather it didn't also require skill points or some similar artificial limitation.
But while trainers work for both non-magic and magic skills, it also makes sense for wizards to be able to learn new spells through individual study, finding the spell in some... learnable form and learning it, maybe after gaining some necessary skill or power for it, depending on how magic is separated in the game.
But there can also be an argument in favor of automatic learning of some skills, but depends how it fits the story and character type. Definitely not the JRPG trope of automatic learning of new skills at certain levels, but instinctive-magic types may just figure out some spells as they improve overall, some characters may pick things up if they see them done by others, and in case the amnesia trope is used, or something about an older soul inhabiting the character's body or some other such connection, there would be moments when memories are triggered.
For the simple general use though, yeah, trainers.

Reminds me of Blue Magic in the Final Fantasy series, which is typically learned by observation or by experiencing the effect. (For example, in FF5, to learn Level 5 Death, you have to be killed by that spell (or be saved by Bone Mail); this is actually a bit tricky since your level has to be a multiple of 5 to be affected, and if your entire party is the same level (which is pretty typical for most of the game), the spell will kill your entire party.)

Tangledeep has a job that learns abilities by observing them being used, while apparently Serment: Contract With a Devil has one main character choice who also learns abilities Blue Mage style.

By the way, here's the requirements for learning Blue Magic in each (main series offline) FF game that has the mechanic:
* FF5: Must experience the effect. (Doesn't count if cast by releasing a monster.)
* FF6: Must observe the effect. (Note that the otherwise non-functional blind stats will prevent this, even in versions where it is otherwise non-functional.) Unlike FF5, it counts if it's a party member casting the spell (there are a couple ways this can happen). Only one character can do this.
* FF7: Must experience the effect (though I think having it miss might be OK, unlike FF5). Note that Enemy Skill materia must be equipped, and it's that materia that gets the spell, not the character.
* FF8: Need item drops from specific enemies. Only Quistis can learn them, and they're only usable as a Limit Break.
* FF9: Quina must Eat the enemy (yes, you read that right) to learn its blue magic. Quina is the only character who can learn such spells. (Also, props to Square for including a non-binary character in this game!)
* FF10: Kimahri must use Lancet on an enemy. As in FF8, these spells are only available as Limit Breaks (called Overdrives in this game), and only Kimahri can learn them, even though other characters are capable of (eventually) learning Lancet. From a standpoint of this topic, the game uses a unique Sphere Grid mechanic, where you use spheres on specific spots on the sphere grid in order to learn techniques and spells.

I like it unrealistic but convenient - just go shopping for your spells or abilities and buy whatever you want, whenever you want, well, maybe with some light tier conditions for the more powerful ones, like in the Divinity: Original Sin series. Although, now that I think about it, that was connected to distributing points on level up as well, as a precondition, so I guess just choosing stuff on level up is equally fine.

Depends on the game really. In a simple straight-forward dungeon crawl I really don't care that much.

Games trying to build a plausible, immersive world on the other hand... there it's nice if there's some plausible way to learn spells - or at least the basics.

I could imagine a system where spells are essentially broken into "components", like an element (fire, water...) and effect (fly, touch, area) and to "unlock" them a character has to learn them somehow - teacher, book, even observation - and then those can be combined and improved by practice (plus maybe trainers).

Well, I haven't found any method of learning spells that actually works. Please let me know when you find a way to actually learn magic!

I like the Castlevania Aria / Dawn of Sorrow way of every enemy type having a soul which will give you an ability if you happen to absorb it (random with different drop percentages for different enemy types).

I always forgot to prepare spells in my d&d playthrough... although I like vancian system

best is when you make human sacrifices and devil gives you power

But that's what a classless system is all about. Even if you are, say, a warrior, you are that because you chose to be one, not because your character sheet says so. Especially with a system where you improve what you use. Class-based systems tend to have pretty uniform members of any given class - the opposite of individuals.

Personally, there are two things I don't particularly like about that system, and they're separate issues:
* You need to prepare the specific amounts of spells you plan to cast in advance. It feels like you need to micro-manage ahead of time, and you don't have enough flexibility to spontaneously choose your spells on the spot. I personally prefer the approach used in some SaGa games, where you have to equip the spells you want to have available in advance, but don't have to allocate your JP (or WP for weapon techniques) to them in advance.
* The segregation of magic resources by level never made sense to me. Why should Cure Light Wounds and Bless use the same resource, when Cure Serious Wounds uses an entirely separate resource? (I would expect CSW to use the same resource as CLW, just more of it.) Why should a character be too spent to cast a weak spell, but still be able to cast more powerful spells? (Some games that use this system also have problems where higher levels might have no useful spells; you might have a powerful 3rd level spell like Fireball, but nothing comparable at 4th level.) Sort-of reminds me of SaGa Frontier 2. The actual learning of spells comes from trying to cast them in a duel or using related spells in party battles (though once learned, it's learned for everyone who can use magic, which is all but one character), but to actually cast the spells, you need to have equipment to provide every type of anima the spell needs. For example, Bushfire requires Tree and Flame, while Life Water requires Tree and Water. Your stone knife doesn't provide either but it does provide Stone, which some other spells use. That steel sword you're carrying, however, doesn't provide anima, and will in fact make your spells less effective (but not to the degree the story seems to imply).

I've come to like the way growth is handled in some SaGa games, namely 1/2/3r/Frontier. In these games, the system is classless, but you have different races (3 in 1, 4 in 2/Frontier, 6 in 3DS), each of which has different rules for growth (well, 3DS does have less variation, but it still has some, especially with Robots and Monsters not getting most of their naturally gained stats). You might have humans who might gain stats through practice, but you have other races, which gain stats and skills in different ways.

One could also implment a rune-system. This would get rid of the learning altogether - you need to find the rune for the effects and stuff, and once you have them, you can cast the spells they combine to.
For growth the runes themselves could grow in power the more you use them. Additionally the character could have a generic "magic talent" to improve rune use (or "mana"). Maybe some high-level runes would require more magic talent to use at all.

SaGa Frontier does something like this. After a battle, if you killed at least one monster, you can have one of your monsters absorb that enemy; doing so gives the monster a random skill (with some more common than others) from the skills associated with that monster. Learning skills this way can lead to a transformation. Robots have a similar mechanic, but enemy robots are far less common and no transformation will occur.

There is the danger of making things too random; basic abilities should be reasonably common (100% chance for some of these later on isn't entirely unreasonabe), and advanced abilities shouldn't be too frustratingly rare.

Sounds interesting. I could imagine that robot-like characters would need to upgrade soft-/hardware to learn new stuff (or even replace abilities), while some monsters maybe "learn" by eating somebody who can do what they want to do... :-)