Its a problem I used to face all the time, I liked an artist but couldn't find out other music that is similar. Sure there are some services out there which will link artists together for being similar, but most ive tried have been crap (every single punk band is linked to Green Day.)
Another problem is you dont want to go out and buy an album just from what someone told you on the internet.
This is where Pandora comes in
This is an awesome site, you just type in the name of an artist or track you like, and itll generate a radio station with similar tracks on it. Its free although you can pay to get rid of the small ads, but they arent intrusive at all.
You can decide whether you like a track or not, and the playlist will change according to your preferences. Its great to find bands you like but have never heared of.
Too bad it wont work on the PSP since it uses flash player