Three general techniques (not necessarily in order) that I use to get more "life" out of MIDI tracks are:
1. I adjust the velocities of the individual notes to give some accent to the notes that I feel should punch through. The amount of accent varies with the phrasing of the line. Some notes really need strong accents and some, usually in the middle of the line, require somewhat less. Don't just increase the velocities of accents. Also look for notes which should be a little subdued compared to the others.
2. Alter the note start times and durations. Real players tend to anticipate and set a the mood with a line by either "pushing" the beat by playing just a little ahead of time or "drag" the beat by playing some notes a little late. Some "groove" algorithms attempt to do the same thing, but they usually sound to predictible to sound like the real thing.
3. Real instruments sound different at different playing volumes. This translates in MIDI to varying sound at different velocities. The line in question may sound much better with the overall track volume set higher (or lower) and a track velocity compensation (a track velocity that's added or subtracted from all note velocites) that is lower (or higher). The resulting volume of the sound may be about the same (since track volume is higher and the velocities are lower, or vice versa. The effect is like a musician playing more softly, but with the channel gain increased.
With certain orchestral sounds this can make a great deal of difference. For example, a french horn can have a pronounced and obtrusive "blat" when played hard. If a softer sound is desired, then I lower the track velocity and raise its volume. That way I get a softer sound and still get the volume I need.
This process can take quite a bit of fiddling, so it's a good idea to make a copy of the track before starting. In fact, you can make several "takes" to compare.
For some examples of music that I've made with these techniques, take a look at my site (Dead Link Removed)
I try to use little or no compression with MIDI instrument tracks. When I do want more punch from a bass track, I usually set the compressor to have a moderate ratio and fairly slow attack. I set the release to be just fast enough to reset the compressor for the next note. The idea is to let the initial transient through and then compress on the main part of the note. Another thing that you might try is using different bass patches. Some sound much punchier than others. Layering can also help to get a fuller sound. In the end, if I can't get what I'm after, I usually go to a real bass.