I use toothpaste and a tooth brush to clean bodies before painting. I then rinse with distilled water and dry with a lint free towel. I find that the toothpaste is slightly abrasive and gives some "tooth" for the primer to hold on to.
I too use this meathod. I like Arm&Hammer as it is not harsh on the plastic and has baking soda so it scrubs nicely. It is also what I use on my second to last paint coat as it works like a finnal wet sand and the last coat just shine