I've had this with every pregnancy too, and with most pant styles.
There are a couple of solutions. You can get maternity pant suspenders that clip to your bra. (Now everything can sag together! Much less noticeable, right....?) Google will find some for you if you want to try that route.
I've had some reasonable success with belly bands. You can get them at maternity shops, and online, or even make you own if you sew. They're just a band of stretchy fabric that goes over top of your pants, and adds a little extra snugness to keep them in place. They're also great for when you're just getting out of your own pants, but not quite in maternity pants. You can unzip your pants and cover them up with the belly band. Works well for postpartum too. And, they make a nice cover-up for your belly when you're nursing - keeps that poor post-partum belly covered up when you lift your shirt.
I got the "store brand" ones before, but they stained and frayed and raveled, so I'm going to get the better quality ones this time around. Check out the BellaBand by Ingrid and Isabel.

Hope that helps!

Oh - I've also gotten desperate enough to safety pin my over-belly pants to my bra....