1. Setup a special downloads folder and have it sync to your iPod/Zune/whatever (this is optional, but makes steps 2 and 5 easier, in my opinion...it also keeps your downloads separate from things you *know* you like).
2. Tag all the files in this folder with "DL" (or whatever you prefer) in the comment field (I use an app called MP3Tag for this step).
3. Setup a smart playlist in iTunes with the rule "Comment contains 'DL'"
4. Occasionally listen to your downloads playlist and decide if something's worth keeping or not.
5. If the album is good enough to buy, BUY IT! If not, just delete it from your library, because chances are 6 months from now you probably won't care about the 1-2 songs that you liked in the first place.

All of this is contingent on taking a hands-on approach to managing your music library and not letting iTunes/Winamp/whatever do the work for you. I've found that this has worked well for me (at any given time, my DL playlist has ~500 songs in it and I try to keep it that way).