For some of us, the tapes and "belt" of the diaper is enough! Depends on your body shape and attachment technique - one of those you can control. Some diaper wearers would probably be best served by suspenders, lol. Otoh, simply wearing pants can help.
If my diaper actually starts sagging it's one of three things:
1. The belt of the diaper got loose, either through stretching or tapes mysteriously migrating.
2. Padding fell apart (more extreme physical activity or just cheaply made diaper)
3. It's way too wet and needed changed anyway - although some more recreational wearers are after the bulk and extreme absorbancy / most wetness.
Many will probably recommend snap crotch shirts (onesies), this puppy doesn't recommend them or underwear because they can be detrimental to diaper absorbancy and can actually make the diaper leak before it should. Anything restricting on top of your diaper can compress it and limit it's ability to quickly absorb. I also haven't found (or tried really hard to find / commission) a onesie that was loose enough in my diaper area to not compromise the diaper. They're great in theory if your worried about diaper peekage, but my tall torso means the ones I've tried are tight in the crotch leading to leaks that wouldn't have otherwise happened.