To echo what Jared said a bit, don't play just to impress. Especially if you are a bassist. Think of it this way, if wanky guitarists didn't have bass players, they wouldn't know where they were at in the song, everything would fall apart, blah blah all that other stuff we are suppose to do.
It make not be as impressive, but making music that sounds good is impressive enough for me.
I'm guilty of slapping too much, but I don't do it a whole lot in a band situation. Slap is more... fun... to me that anything. It's not very practical, you rarely find a guitarist/bandmates who know what to do when you start doing it... basically (ha ha bass), get solid before you get flashy.
Good luck on bassing though. Sorry for derailing the thread.
Primus is probably my favorite "slap band." Les Claypool is flashy
while remaining solid. Some might aruge he's "sloppy" but I mean solid in the riffage sense.
Also, there is no better place to learn about something than to go back where it started. Larry Graham of Sly and The Family Stone invented slap on the electric bass. If I'm not mistaken, "Thank You Falletin Me Be Mice Elf" (I'm sure I butchered the title) is the first slap song they recorded. I also downloaded a really cool bass solo by him that was mainly octaves but it was so cool and groovin'.
Mark King of the band Level 42 is also renown for slapping. His bass solos are simply amazing.
