Don't rush, or force it either. I only use reverse kegels when I stretch, and I know guys use it when they climax, but you need to have good practice with it.
The last thing you want, is to strain when your doing this.
Search for the thread by DLD on kegels, and it discusses the conventional kegel, followed on by the reverse. By practicing this, you will know the difference in isolating them.
Me, personally, I only recommend the reverse when doing length work.