ugh, sweatpants are gross!
Seriously, best bet, in my mind, is develop some smart shopping skills. Maybe check out thrift stores, maybe check out chains that clear off overstock and the like (up here there is a widespread chain called Winners that does that, don't know if they operate down there), and don't turn your nose up at WalMart or the like.
When buying new things, pants with some stretch--either all over, or hidden elastic in the waistband--will be more forgiving in a change in your size. And of course, if anything buy a little loose and wear a belt, if you expect that you'll be gaining more. Things like cargo pants are often a looser fit in the seat, and this will also make them more forgiving than more form fitting clothes (the worst for fit in pants is usually while sitting, when your butt is pushing out towards the back and your gut pushing out towards the front. Things with space in the butt can shift to follow your gut more than can something which is also stretched out in the back)
“I think, at a child's birth, if a mother could ask a fairy godmother to endow it with the most useful gift, that gift would be curiosity."