Any full, complete player knows his arsenal inside-out.  Unlike classes like the soldier or HW guy, medics actually juggle three main weapons, along with their grenades. 

     The medikit is your PRIMARY weapon, make no mistake about it.  You will have this weapon in your hands over 50% of the time you play.  Why, you might ask?  A dying teammate runs by, being chased by an enemy.  Hit you attack key and boost his health up to 100% in a snap.  Or, better yet, if an enemy flys by, you can infect him.  This will A) piss off your opponent off and B) eventually kill him (most likely). 
        The medikit is simple and easy to use.  Just go up next to the player, touch your fire key, and the kit will heal/infect instantly.  There is strategy for the medikit, but not much.  First of all, don't try to infect soldiers head on.  Although they are not fast, soldiers can easily fire at your feet, sending you flying backward, and then shoot you as you land, turning you into a pile of fresh gibs.  Even when soldiers are concussioned, they can still fire at their feet and avoid your kit.  My suggestion is to just avoid them and use your speed advantage.  If you really have to infect a solier for some strange reason, then run away from one, hide behind a corner, and infect him as he turns the corner.  Another trick I picked up was to avoid a soldiers rockets by jumping around and running back and forth.  If you are careful you can count the rockets he shoots at you.  When he gets to 4, he will have to reload, so try to rush him and infect.  Be aware that this tactic has a higher gib rate than success rate, but it is still worth a try.  Other than soldiers, infet every CLASS!  If you ever get real desperate, dont forget that it can be used to spam.*ugh*
    There are a couple of situations where the medikit is a real plus.  On ANY of the the territory control maps (dustbowl, warpath, CZ2, etc.) if you manage to infect the enemies that are guarding a specfic point, you and/or your team will be able to take the point with ease.  Try grenade jumping past the initial defense in dustbowl.  You should be behind the enemy line, and able to infect several teammates.  Unless they are smart (what you play public servers?  forget that last line) they will spread the disease and your team will easily breakthrough and win the map.   

If you read the intro to this page, you can recall that I said you would juggle three weapons.  Well, there is four in your arsenal, and this is the one you DONT use.  The single barrel shotgun is weak.  It lacks fire power, isn't that accurate, and it has a weak sound.  You could use it when, well, nevermind.  Dont use it.  This gat sucks.  If it wasn't part of the medic, I wouldn't have wrote this. 

The double-barrel shotgun.  Long has it been a fan-favorite, but now I can truly give it credit as the MedicS GaT oF ChoicE (R).  No really, the shotie is the gun to have.  The shotie packs a lot of punch.  2-3 well placed booms will easily down a scout, sniper, or engy.  Anywhere from 4-7 booms can easily down another medic, a spy, a pyro.  Sure, rapid fire (supernailer) is nice to have, but if you become accurate with the shotie, a rising kill count will follow.  The shotgun really come alive underwater.  If you confront a soldier underwater, 95% of the time they will stick with their rocket-launcher.  Great!  Using your strafing speed, put round after round of your 2barreled shotie into the soldier.  He'll drop a lot sooner than expected; you know why?  When an enemy is underwater facing you, their body is laying parrallel with the bottome of the water.  Their head is facing you directly.  So, their isn't much to hit, other than the head, which triples the damage. Dont you love using the technical aspects to your advantage? 
Another thing I love about the shotie is that you can steal kills.  I mean, how many times have you weakened an opponent to less than 20 health.  Some HW comes around the corner.  He starts up his AC, and bags the kill as you fill nails into the enmey as well.  When an teammate HW is shooting at an enemy, ty to get real close to the enemy, and put a well places shotie blast to the head, since the shotgun packs such a powerful punch, you might kill the enemy.
O yeah, here's another advantage with the shotie.  Is it easy to run out of ammo with the supernailer?  All together now: Yes.  Is it easy to run out of shotgun shells (provided you picked up the ammo bags in the respawn)?  Is it frickin possible?  Yes, it is, but it is very rare when you run out of shotie shells.  The only thing you have to worry about is keeping your shotie reloaded.  Since it says it has 16 shells in a cartridge, 10 shots left may seem like  a lot.  But In reality, 10 shots is only 5 (each burst takes 2).  Keep that in mind.