It's not as simple as it seems. Everyone has a different taste in music. Take me for example; my taste in music varies greatly. I can go from the most hardcore ICP (Insane Clown Posse) song to the softest Jazz song out there (and believe me when I say those two genres are complete opposites).
But to create an awesome playlist, you must first identify what type of playlist you want, i.e. what type of music you'd like on it. Then you must choose which website you will use or if you have an application downloaded, which application. I personally recomend playlist.com for online things and iTunes for applications.
Now. Pick your top three artists, like, those you listen to the most or those you like the most. If you have a wide variety for taste, then simply pick the three you listen to the mose (as those are most likely your top 3). Next, it's as simple as picking out the songs you listen to the most out of those artists and putting them in some particular order.
If you'd like a wider variety, expand your search to your top 5 or top 7.
I personally have my ordered by artists then song. My favorite song by my favorite artist comes first then we move on down the list to my second favorite with my favorite song by them. And so on.
Well I guess I was wrong, it can be pretty easy when you look at it in this perspective.