Mint does offer an option to "Roll over balance at the end of each month". That means if you budget $100 for clothing but only spend $50, you'll have $150 for next month, or if you spent $120, you'd only have $80 next month.
Admittedly though, YNAB's strategy of budgeting last month's income may suit your needs better.
I don't mean to hijack your thread, but if you say you're an experienced Mint user, perhaps you can answer my question. I recently transferred some accounts and in the process there was a short lived computer error that showed I had 2 accounts containing $X when I only had one. Everything shows correctly now, but when I look through my Net Worth history, that error shows up saying my Net Worth was more than it actually was for that month. Is there any way to correct this?
Thanks.