Optimize Combat

Optimize Combat
Spend a lot of time practicing to optimize your combat. You should try to learn to do everything as perfectly as possible during combat so that you (a) win the battle and (b) end the battle in the best shape with health and mana. By perfecting your play (PP) you will become a better player, have an easier time, and sometimes gain respect from other players due to your abilities.

Keys to perfecting your play:

Organize your action bar very well. Put your most used abilities on page one. Put abilities you use less often on page two. Learn to switch quickly between bars during combat (using shift mouse wheel or shift 2). Learn to use the hot keys for your action bar to cast spells more quickly. You may also enable extra action bars through the Interface Options menu and bind them to keys using the Key Bindings menu. This may help you avoid having to switch between action bars during combat.
Learn all of your abilities. Learn each ability so that you will know what it is used for and when to use it. If you need help, talk to more experienced players of your class or read class strategy guides.
Get your abilities and spells as soon as you can. A common thing to hear is, "oh I haven't got that ability/spell yet." There are good reasons for this such as not having enough money for spells or you need help on a quest. However, laziness shouldn't be the cause of you not having researched an ability. Some abilities such as healing, resurrection, buffs, and cures are very important to a party.
Damage over Time Spells (DoTs)
After casting a DoT on an enemy, watch the icons by the enemy's title bar to see when that DoT wears off. Then use the hot key and cast it immediately again. Using this method, you can have your DoT active for most of the battle.
Be active during combat
A common mistake is to start a battle and just sit back and watch. You should be using your abilities as soon as they are available. There should be very little time between when the ability becomes ready and when you use it. You will find that making little changes and being on top of things will significantly improve your effectiveness in combat.