I don't think there is anything wrong with asking your peers for their opinion of your work. Also, if you take the time to listen to other people's songs and then make a nice comment on their myspace site, they will often reciprocate.

Include a link to your songs or site with your signature. That encourages people to go and listen to your songs and comment about them. I can't listen to your work (or comment on it) if you don't tell me where to find it.