I know it's gonna sound weird, but try a pair of bib shorts... the lack of a waistband means that there's nothing there to make random lumps around your waist. They have a bit of a "girdle effect," and the chamois never moves unless you move it yourself. I've sworn off plain shorts forever!

Edit: Oh yeah... if you go for bib shorts, get a jersey that zips all the way down, because you generally have to take your jersey off if you need to pee (only downfall, but totally worth it IMHO)