Some things I learned is: Don't sacrifice aim for CPS, if you are getting comboed then run away for a second, practice strafing and aiming, do a variation of "a" and "d" taps along with "w" taps and maybe throw in some block hits. Just don't be predictable.
Whenever I fight I normally do a variation of d tapping with block hitting, or w tapping, or just strafe.
use weakness arrows. if you do not have them then at least use a crossbow as a way to deal more knockback to your opponent. generally, you should try to bring gear better than that of the enemy's unless you do not have it (duh). also, take advantage of strafing, it can throw people really off. especially in 1.9 pvp, where if you miss a hit, the fight's probably over for you.