Minecraft wiki entry for Player_versus_Player, associated text: Don't be scared to act like a coward. If you stand no chance, save what you can save and run! If you're nearly killed, and have no hope of victory? Run! And don't let yourself be lured into 1-on-1 PvP unless you can win!
Use your mind over your in-game muscles! Preferably, use both! Most important is to use your environment wisely. For example: if you're in a jungle biome, run into the trees and attack your enemies! If you're in a big NPC-village with iron golems: make your foe hit the Iron Golem, then run off and snipe the foe! If you're on a small island in an ocean biome: get a boat, sail a little away, and use your bow to take down that diamond-armor juggernaut! While in the Nether, surprise the enemy with a bed and kill them with the explosion! While in The End, spleef your enemy into the void for a sure victory! It will all be yours if you succeed! Your sword might be a strong weapon, but your bow is even stronger if you use your mind. Also, don't be scared to use potions: they can give you a great advantage over your non-potion using enemies.