ORIGINAL: CJaysMusic
If you track the songs correctly, the only mastering you should be doing is fading or crossfading each song and making sure each one is at the same volume.
even if you don't feel like competing in the loudness wars you should at the very least have a limiter on your master bus to prevent clipping. you also need to check that each song's tone color doesn't change drastically from one song to the next. to me, "mastering" consists of the following, at a high level:
1. any effects you put on the master bus for each track
2. making sure volume is consistent across the album (which you said), but more than that, making sure that the loud songs are loud and the soft songs are soft.
3. making sure that when listened to straight through, the spaces between the songs are musical and not abrupt.
4. making sure that the tonal color of the songs is consistent.
5. the process involved in actually creating a CD master which would then be copied to a true CD print master and used to print CDs, or at least the master from which all your other CDs are burned from and labeled with a Sharpie.
i probably forgot something but that's mastering as i've heard of it. most people here only concentrate on #1, and if you are simply going to put up a bunch of random mp3s on soundclick i suppose that's really all you need to do. but you might at least try some #2 and #4 as well.
i suppose if you are recording jazz or an orchestra or a bluegrass band (with a single mic, to keep it real) in a great-sounding room you could probably just print the takes straight to CD or mp3 or whatever but i doubt very much that any hobbyist could pull that off on a pop, rock or hip-hop record and have it sound even remotely professional.