Generally the first obstacle with rap vocals is at the recording stage.
Most MC`s wouldn`t know mic technique if it raped them with a baseball bat.
They have a tendency to lip job the mic all the time in some insane atempt to be the loudest, so make sure you keep em a good distance from themic. Employ a pop guard.

Most Rap tends to be heavily compressed and really flat and upfront.
But heavy compression will sound ugly unless you make good use of a de-esser to get rid of ugly hisses, sibilants and so on.
Also make good use of a noise gate as again compression might cause problems with any recorded noise.

Harmonisers get used quite a lot to thicken up rap vocals as well, generally set off key with a 3-5 voice spread.