Is It a Common Practice to Contact the Reviewers?
My strong advice is a clear NO! 

There are several reasons:


The journal might contact other reviewers, not the ones you suggested.
Possible conflict of interest, even if you don't send your paper with the review request
The reviewing process is a business between the editor and the reviewer. It is specially designed to enhance the quality of the accepted papers (Blind, double blind reviews, etc). Once you contact the reviewers, any chance of a blind review goes up in smoke.
If the potential reviewer that you contact, is not selected by the journal he/she might feel offended.


So, in a word:

 NO