All of these things should be done only after fasting for at least, 8 better 12+ hours. otherwise you run the risk of it coming back out both ends at once!!
I have posted about a glycerin enema that is just about unretainable.
if you want cramping in a normal 2-3 qt enema, Epsom Salt and soapsuds together in the same enema gives you good cramping and a very strong purge. It makes up the first round of most enema sessions for me.
If you want to give it a little more kick, you can take a Fleet Bisacodyl mini enema about 10 minutes before you start, or take 6 Dulcolax or in my case generic bisacodyl tablets about 2 hours before starting the ESS enema. It typically kicks in around the third round.