The key to using bunkers is knowing all the ways you can use them for cover (not just inside), and just when which way is best.
Always remember that if you wait too long to get out of a bunker, there is no way you will without killing everyone outside who is holding you in. This will determine if you go inside for protection, or stay outside and use the bunker as a really big tree. You do not want to be on the outside if there are a lot of guys around, and they can surround you. And you don't wanna be inside there alone, cause they'll just charge and unless you were a decoy, you've made your self completely useless to the team, hello deadbox and angry Captain.
What I'm saying is, don't enter the bunker to fight off a million guys by yourself. Enter a bunker when the odds are pretty much even against the bunker guys and everybody else. This is useful as a last ditch defense of a flag, or whatever item the scenarion may hold valuable to your team, such as a leader, or hostage. Fighting off guys from a bunker is useless unless you need to defend something. Don't hop in a bunker just to get killed in a three on one.
The other time you want to be IN a bunker is when you are fighting between two bunkers. That is, both groups in the firefight are inside their bunker. You have better cover and can discuss what you want to do to get the other guys out, like send a man to their flanks.
Now for scenario number two, when to use a bunker as a "tree".
This is most useful in either retreat or advancing quickly up the field. These are opposites I know, but the principals are the same. You don't plan on staying there, it's just a pit-stop of cover on your way. Just the same way you use the woods when making a move. Duck behind them and stop and fire, or run behind bunkers to reduce the chance of getting hit while making a move, hide, etc...
This can work in your favor when setting up an ambush. If it appears a runner or scout has blown his cover in a scenario (or on purpose for an ambush), he can draw the other team through some bunkers, and into, or even over, a group of his men. They can sit outside on the blind side of the bunker (where the other team can't see them) and bust up their ranks when they come into comfortable range, on signal.
Snipers, (read my sniping page for my definition of what a Paintball sniper is) can use bunkers as ambush points on his own, but this is often a suicide mission. However, with a talented sniper and a good route of escape, this can be done while still preserving the snipers life (snipers can be awful important!).
You arer probobly wondering about how bunkers can help you communicate on the field. While you can't always do this, it's useful sometimes. When stalking, sniping, or gathering recon on a guy or group, sometimes you need to show another player were he/they are. If you have developed hand signals, or you can talk quietly well (sounds dumb, but really, it's a developable skill) then you can use bunkers as landmarks to point out position, and maintain knowledge of their position if you lose them as they move.
That's about it, any other questions on bunkers can be posted to the message board.
-Captain Clay