I have a few gigs coming up with a stack of rather difficult music among them. One piece that I'm working on is Walter Piston's Serenata. It's a nice little piece comprising three short movements for orchestra. The notes aren't hard, per se--nothing a high school bassist couldn't play--and there are no real "licks" in it. The real challenge comes from the meter changes, which occur almost every measure in the first movement. The changes from 5/8 to 6/8 (with occasional 2/4 and 3/8 measures for, well, good measure) make the counting the trickiest aspect of this piece. On top of this, sometimes the 5/8 bars are 2+3; other times they're 3+2. Many times the 6/8 bars are better understood as 3/4 (i.e., 2+2+2).
I happen to have a recording of the piece (on the same disc as his fourth symphony, which I've played and like a lot), so I've spent most of my practice time as follows:
First, I just listened to the piece a few times and followed my part.
Second, I practiced conducting the most difficult excerpts (there are about four of them). Once I had done this a few times, I tried conducting along with the recording.
Third, I practiced speaking the rhythm in my part while conducting. Once I got good at this, I tried counting and conducting along with the recording.
Finally, I went to my bass. With a firm understanding of the meter and rhythm issues, the part came together quickly.
So far, I've spent much more time working on this piece away from my bass than I have actually "practicing" it in the conventional sense--probably 80%/20%. (Part of this, I'm sure, stems from my belief that it's more important for bass players to play in time than in tune. Rest assured: my intonation is pretty good, otherwise I wouldn't be getting gigs....)
I did this same thing with two other pieces that I recall: Bartok's Dance Suite and Stravinsky's Rite of Spring. In both cases, the notes are easy--rhythm and meter are what cause the problems.
It's important to show up to rehearsal knowing your part. That means not just the notes, but also how what you're playing fits in with everything else. Listen to recordings, follow your part and, if available, follow the score. Listen to a few different recordings so that you don't get locked onto one particular interpretation. Figure out what the flute is playing when you have eight measures of rest, or who is playing the same line as you. Sometimes your most important preparation is done away from your instrument.
And the best thing about this kind of practice is that you can do it just about anytime, anywhere!