Party build:
I found casters to be extremely useful. You'll be surprised how effective low level spells like Sleep, Grease, Web, Soundburst and Tasha'sHideous Laughter can be. Note that the first 4 are AoE spells so while some of the monsters might make their save, there's a good chance of taking out at least some. Only Grease and Web affect undead though.

My favorite tactic with Grease is to move a melee character next to a tripped enemy. You'll get a free attack when they get up from prone position.

At higher levels Stinking Cloud is very effective. Sickened monsters can't make attacks, use items or potions or attack. Undead are immune though.


Making (near) infinite money
This tip exploits item creation. Make sure all your characters have at least 1 level in a spellcasting class and at least 1 item creation feat. (Wizards get scribe scroll for free)

Have one character max out the Appraise skill. High Intelligence helps. Rogues and Bards are best. (Equip the best Headband of intellect you can make for maximum effect.)

Creating items cost a little xp and half the price of the item. (the price listed in the Dungeon Master's Guide) Your actual sale price is modified by the appraise skill of the character who's talking to the vendor. Early on it's easy to get a little more than half the price of an item which will allow you to make a small profit. Later on you'll be selling items for 90%+ of the value while paying only 50%.

The downside is that if you wait too long, xp from encounters will drop hard due to level difference between your party members and the monsters. I found staying at level 6 works very well. Just burn up all excess xp while making random magic items and sell them for profit. Go out and earn some more xp and repeat. You'll be surprised at how fast you can earn enough cash to allow you to craft or buy anything you like.

Nice idea!

Easy Money
In the northern church in Hommlet on the second floor there is a bookcase which holds several high value scrolls and potions. You can sell them for some quick money, but you should probably spare a single raise dead and heal scroll.

Use summons to flank opponents
Not only they provide good cover, you can summon them behind an enemy's back to get that flanking bonus. Cleric's spiritual hammer is especially hard to destroy.

Use crafting
Crafting items is more of a late game thing, but you should definitely scribe some scrolls with your wizard, who gets the scribe scroll feat for free. Spells like grease can be extremely important, and if you run out of memorized spells, you can just cast them from scrolls (though their effectiveness is somewhat lower).

Use charge attack often

Don't forget about the "Wait for" command
Sometimes it's more effective to just clump together and let enemies come to you.

Use Read Magic to identify scrolls/potions
As opposed to Identify, Read Magic doesn't cost any gold.

If you are going for range weapons, Precise and Point Blank shot feats are essential

Bucklers don't give a penalty to attack if you are wielding a crossbow/bow
Good for wizards and its ilk.

Reach Weapons are THE 2-handed melee weapons in this game

Egh, I have too much advice. Ah well, this is a tips topic, so you'll just have to suffer!

u]Spells:
Knowing which ones to pick for which situation can make fights loads easier. Such as...

A successful Charm spell can turn the tide of battle. Charm a boss, have him fight his own minions. Spam it and charm half the combatants! When the battle is over, conveniently walk the charmed into a corner and pummel them to death.

Tasha's Hideous Laughter (or some form of making prone) is good for a surrounded opponent. A bonus to attack, and a free strike against them as they stand up. Plus sneak attack damage!
Also, I found Tasha's + Web absurdly useful, since it usually keeps them prone.

Mage's Armor adds nice armor to anybody who doesn't wear any, including monks.

Sleep is handy for swarms of little guys (goblins, kobolds).

Fireballs do awesome damage.

Hold Person is dang near cheating, since you can Coup de Grace a held person, for what's often an instant kill.

Having a Good cleric with spontaneous healing is incredibly useful.

u]Tactics:
Charge attack to get to enemies typically out of reach, especially flat-footed ones. Run gets you around the battlefield fast (up to 4x faster, I think) at the sacrifice of your action.

5-foot steps are good for taking a step back out of melee to cast a spell or fire a bow.

Use ALT in combat. Holding it while hovering the mouse over a location shows the path you will take, and little sword icons for attacks of opportunity against you. You don't have to take the default path! Make a better one.

Body block enemies with your fighters. Keep enemies away from certain targets, pin them against walls, etc.

Oh and if you constantly miss in ranged combat, it may be because you get some bad penalties for shooting: past an obstruction/over a person's head, at something that's in melee combat (pretty much half of everybody), too close to a target (unless you have Point Blank Shot). The penalties add up.

And so forth. That is all.

A long weapon, like Glaive + combat reflexes + cleave and great cleave.
With thesa a skilled fighter becomes an epic weapon of mass destruction.

These are good tips. Thanks. I'm consistently getting my arse kicked.

Learn to enchant weapons.

Cleric with Good domain can (with enough levels and craft magic arms and armor feat) enchant a weapon to Holy.
2d6 extra damage against evil.

Cleric with Law domain can (with enough levels and craft magic arms and armor feat) enchant a weapon to Axiomatic.
2d6 extra damage against chaotic.

Cleric with both Good and Law domains can do both!
A weapon enhanced to +3 and 4d6 against chaotic evil opponents is downright awesome in the hands of a skilled fighter.
Particularly as... well guess what kind of opponents you'll be meeting in the Temple of Elemental Evil.

Give the same fighter a Girdle of Giant Strength
(you need to be able to cast bulls strenght to craft this) and watch those opponents just DIE!