The compression on rap vocals is vital to the sound and success of the song. There are a few techniques that you can use, but I'm going to share one with you that is popular with my clients. Duplicate you main vocal track or if you have multiple main vocal tracks, bus them to an aux send. On this new track, add a compressor that has controls for attack, release, threshold, ratio, and output gain. I prefer the Waves Renaissance Compressor, but you can use any comp that has similar controls. Set the attack and release to be very short. Play the track and bring down the threshold until you start to have the compressor engage. Now drop your ratio down to around 8:1. This will squish the track and give you a nice even volume level for the vocals. Adjust the threshold and output volume to get a nice balance of compression and volume.
Now play this track along with the original vocal track. Keep the original vocal track up in the mix. This is your main vocal track that need to be up front. The new squashed track is mixed into the background. Bring the level up slowly until you hear the overall vocal level filling out the space in the mix. This technique keeps the vocal up front in the mix regardless of how much presence the vocalist put into the recording. This even tone of vocals will give your rap vocals a polished sound that is professional and captivating. Now you may need to adjust levels and ride the faders, but done correctly, this is an easy way to get big vocals into a bass heavy mix.