1. Do lots and lots of PVP. BGs, Arena, Duels, World PVP. Everything you can to learn the abilities of other classes and what the effective counters are.
2. Get better gear (which is a side-effect of no. 1).
3. Use keybinds instead of clicking. You need to be able to hit the keys without looking at your keyboard or your action bars (infact, remove those). I fail at this

I'm a BG scrub I both hate and suck at arena but I love BGs and world PVP. But I am sooooo much better than I was when I started. It's all down to item no. 1.