There are a few good tactics to PvPing,
Strafing: Do quick circles around your enemy while focusing and hitting the target at the same time.
Pots:Bring strength 2 for every fight to get an advantage, bring speed 2 for an edge on your enemy, or a quick get-away.
Weapons/Armour: Do not put any enchantments on close range weapons other than sharpness, other enchantments will weaken the power of your sharpness.Bows do not have the same qualities as swords and axes, so go all out on the enchants. Go for protection 4, unbreaking 3 armour. You could use thorns to cause your opponents armour slight damage with every hit they take, however with protection 4, unbreaking 3 and thorns you can't repair your armour in an anvil.
Crits/jumping: Do not use this tactic ever, you take more knockback in pvp (you don't want to be knocked back in pvp) and you do more damage, but your opponent also does more damage to you, so it's counter effective.
Don't get Shot: If you are in pvp with a good archer, you need to dodge those arrows. Run in a zig-zag pattern to throw off their aim.
Don't let your armour break: If you can tell that your armour is weak, Run! The worst thing that could happen in pvp is losing a helmet or boots.
Mcmmo: If you play on a server with the Mcmmo plugin, be sure to level up your skills in this order. 1.Unarmed 2.axes 3.archery(only if you're good at it) 4.Acrobatics.
Identify Hackers: wondering why you are getting hit while a player isn't looking at you? Well those are hacks, and they are very common in pvp, eating/drinking and hitting at the same time is not possible withought a hacked client. Somebody head is twitching and snapping from player to player? That's a hack. Don't use hacks yourself.
Gapples: if you don't know what a gapple is, it's an enchanted golden apple. If you have them, use them. Don't waste gapples and exhaust your resources. If you have a decent gold farm don't be shy to use your gapples.
Hope these tips will help you ;)