Here is a "how to" and some rules that will help mixes not suck and make this fun & easy for anyone. (thanks jjlook!)
1. Put your mp3 player on random play. I'm using itunes (I'll assume your mp3 player has some kinda similar random play option,) so I set my mix to Party Shuffle. When I do this I can see what the next 10 or 20 tracks are gonna be, and if they're gonna be something really horrible, i just shuffle it again.
2. Listen to the mix. Wow, fun already. I think about 90 minutes is perfect because that's how much music would fit on a Maxell XL II when I was using such media.
3. Weed out the junk, which in my case means audiobooks, comedy records, horribly long tracks (longer than 20 minutes is probably too much), horribly short tracks (except Fishstick Gumbo or similar). You can do this before listening if you can see the playlist up front.