I asked this question not that long back and got some good advice of rythm tec. basicaly compression (pre and post recording) to keep things on the same level - When i was doing this i used quite heavy compression as my rapper (joe) was moving around alot and it was quiet in parts and loud in others. and a touch of reverb as the vocal booth i used was quite dry.
another thing to concider imo is to try and get the vocals to fit into the mix of the track............
Not much to go on but hope its a good start...
Try and find my post on this further down the page as Rhythm tech put it in a more understandable wording