Put the butt plug in your ass, then jerk off. Leave it in after you orgasm, and go outside and do something for an hour while it's in you. Then, come back inside and jerk off a second time with it in.
The use of a tight fitting pair of spandex shorts can help. You'd probably be best off buying women's, as they tend to have the seam running up the back, which will hold the toy in better.
If you buy a pair of Champion brand that have the diamond crotch in it, you can actually cut that diamond area out and pass your cock through it while pulling on the shorts, and now your plug will stay in place.