Here are some tips that served me really well:
The most important thing is to build your base like a evil genius!
You could of course build a normal base and kill everyone who tries to enter, but thats just evil and hardly genius, so listen up my little evil geniuses in training.. this is how its done!
Build your base in a fashion where it looks like a normal company ( that is tasteful enough to put their HQ on a volcanic island of course. ).
This means, avoid putting any heat generating items near the entrance.
I suggest the following:
Make one base entrance, 1 star, completely open.
Inside a corridor which goes to your sleeping quarters with a 2 star door
Put a whole buncha bunk beds in there, and a door on security level 3, in the space behind the door, put a whole bunch of lockers only.
On the other end of the corridor that lead to the sleeping quarters, put standard things such as a mess hall and other default non heat generating things.
Then put your evil base buildings at the end of those legit buildings.
This setup will help a ton. Agents will land on your island, enter your base, see the locked door to the sleeping quarters, enter it.. see the high security door, and try desperately to break in. But as long as they don't find anything incriminating, they will leave which actually LOWERS your heat rating ( investigation revealed nothig suspicious after all ).
Additionally, put a whole lot of topside shacks up, put sleeping quarters in them with some lockers, and a 3 star security door. Agents will waste a lot of time breaking into them just to find 2 smelly lockers. =)
Speaking of lockers, always keep enough to have the maximum quantity of minions. I also recommend putting the control room in a much frequented area, then put your evil genius in there, so he constantly gives bonus benefits to the people working in the control room. Occasional executions help as well.
Make sure to keep artifacts / rare theft items hidden away safely in the illegal part of your base.
Also make sure to abuse the cloning vats liberally. A cloned evil genius that gets killed lowers the pesky attention you receive by quite a bit =)
And of course ..don't forget about a trap filled tunnel with a security 3* door enclosing it, to laugh manically at curious agents ( not really effective, nor needed but fun !! )