here's a few tips....for rappers, put them in a damn straightjacket and tie em down cuz fuck, most dudes have no clue on how to not move around in the booth....stop wildin out unless u want a really shit recording lol
but on the real, use a very light compressor on the vocals....gets rid of most quick jumps in volume..thats a good basic tip, but try messing with different settings, and try several passes so u can get the perfect setting...and remember, every artists is different so you gotta make an effort to know your artists!!!
also, a really cool technique for recording singers, especially ones with a really strong, powerful voice, try micing them up twice......suspend a nice condensor in front of their mouth, then aim a dynamic (basic sm-58 or 57 will do fine) directly at their throat, and keep it CLOSE!! (almost touching their throat) You get a really nice sound out of it if you got everything done right....try moving the mics around (different angles etc) if u get some phasing issues, but most likely you should be fine...also make sure you got your pop filter in front of the condensor...and if you have any sibilance (ess sounds), just throw a de-esser on there to clean that up nice
most of all, just try to be creative and try different things....sometimes you'll be quite suprized with your results!!