Syringe is Your Strongest Tool
This one may seem a little obvious to those of you who want to learn how to be a Medic, but you would be surprised to find out how many Medics are just playing the class to try some new rifles. If you do not want to play the objective and your class role, there is that other military shooter out there you should go play instead.
Always keep the syringe equipped. There will be moments on certain maps you feel the rifle grenade may be necessary, but trade your health packs for the grenades instead.