Medics are not heavily armored which enables them to be quick. The advantage of this is that they can outrun any opponent except a scout. Try to launch an organized offensive with heavily armored players as Medics can use a Demoman or HWGuy as a bullet shield. To do this simply duck behind the player and use you medikit to heal any hits they receive from the enemy defense. Try not to lose you teammates when roaming the enemy base.
Medics are fast enough to preach a little death before heading to the flag room.
Keep in mind that because Medics are so fast that you want to be careful not to outrun your teammates who are providing you with fire suppression. Once you have the flag, try to let your teammates do most of the dirty work. You will want to stay out of enemy sight for the most part, however, you should never pass up a convenient opportunity to give any enemy an insidious disease. The flag makes you stick out like a sore thumb and even the weakest players will try to stop you. The beautiful thing about action packed games like TFC is that you can play as if it were a war of atricion.