It took me a while to be comfortable enough to be use slap in a band. But, I find it now very rewarding.
A few things that I have leaned over the years:
1) Some basses cannot be slapped.
Some models won't work. So, ask around and check your bass is OK. A Jazz Bass will be easier than a Violin Bass, etc...
2) Setup is important.
Strings too low and they rattle. Strings too high and it becomes very hard to pop.
3) New strings sound amazing.
I've made the most progress with a set of new strings, when you get that nice metallic bite, and when they bounce effortlessly.
4) I second Paul 5: focus on clarity and timing
5) You can slap anywhere.
On the bus or on the train: hit your thumb on something hard, say, while you listen to Level 42.
... you'll get a good sound once you get that bounce right ... so, bounce that thump through the day to build velocity and stamina.