Step1
Stop worrying about what you "should" do. Whether new to the game or an old burn out, stressing over getting the wickedest tier 4 raid armor before having to go to bed, or back to work will exhaust you. Reading hour after hour, and page after page of game guides, and how to guides sounds more like work than a game. Aren't games supposed to be fun? Don't get me wrong the guides are great for reference, and make playing the game easier when you get stumped. If you're in a guild that pushes you to play for hours to keep up with the raid group, and you're getting burned out, find a new guild. There are lots of very laid back guilds that love to just have fun.
Step2
Get involved in the plots, and story lines of the game. The scripting of Warcraft is actually very good. It is sometimes humorous, or sad, but almost always compelling. Mindlessly completing quests, or just farming for the XP gets boring. Many times paying attention to the script helps you figure out what is happening later in the game, and gives insight into how the racial, and cultural dynamics of the game world work. Players with better knowledge of why they are doing a quest for some faction generally level faster with less effort, because what they are doing makes sense in the game to them.
Step3
Make friends, join a guild that fits you. Spend an hour standing around in Stormwind, or Iron Forge etc... just talking, joking around, and enjoying the interesting people from all over the world you meet in WoW. You'd be amazed how much easier it is to get in a raid group after taking a genuine interest in someone, rather than just "bull dogging" for an invite.
Step4
When you get frustrated, or even burned out on the game. (here's an amazing concept) QUIT PLAYING!!!! Go out to dinner with your loved ones, see a movie, engage in real life for a while, and forget about WoW. It will still be there when you return. I have stopped playing a character for months out of frustration, and vowed to never play that toon again, only to come back, and play it with a fresh perspective, and made that character my new main. I have abandoned quests out of frustration, and picked them up again later, and completed them with ease after stepping away from the game for awhile.
Step5
Bottom line: WoW is many times more enjoyable in moderation. I used to spend 14-16 hours a day 2-3 days a week on WoW when I first got it, and got so burned out I nearly cancelled my subscription. Now I play truly for fun, and it is really the most fun I have had playing any game ever.
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