Us hunters have a slight advantage in rare hunting while leveling! The lovely spell which helps us with that is "Tracking". If the area you are currently questing in mainly consists of Humanoids, make sure you have your spell "tracking Humanoids" active. If if consists of mainly Humanoids and Beasts, that's what you should be tracking! Although it clogs up your mini map slightly, it is a great tool if you don't want to miss a rare hiding in a tent/cave while you are questing outside!
<Mousing over my minimap, there is a different named mob nearby 'Grizlak'.>
Every new questing point that you head to with enemies, scan over your mini map with your mouse, and check for any unique names. Rare mobs will be named differently to the majority of creatures around it. When you mouse over the unique name, head towards that direction and hunt down that rare mob!
<Misty doing a fine job distracting the mean rat!>
Another tool that will become highly useful in the higher levels is the addon 'NPCscan'. This can be downloaded from curse.com and is pretty much a must once you start questing in the Outlands & beyond. It notifies you of most rare mobs within a certain radius, and creates a loud sound along with a screen pop up.
Along with the above tips for hunting rares, remember to keep your eyes open, and if there is a mob that stands out from the rest (larger in size, slightly different appearance) this may also be worth selecting to see if the silver portrait comes up around it's nameplate.
Rares will also award you with more XP than standard mobs, and usually a better than average piece of loot.