I get asked that a lot. First of all, I'd like to define "need." I don't need new shoes. The shoes I have may be old, they may be out of style, but they fit and don't have holes. I don't need a new couch. It actually does have holes, but I flipped the cushions over. I don't need most everything, I have everything I need. So that's 90% of the battle right there.
But today I needed diapers, and toilet paper, and a birthday present, and window cleaner, and I saw some good deals at Target (3 for 4$ Goldfish crackers, minus the $1 off 2 Target printables, making them 4 for $3.33, in case you're wondering) so I went and I spent $50 extra. I used money I was planning on putting towards credit card debt. It's unrealistic to think that I will always be able to stick to my budget. I mean, life happens, birthday parties come up and kids use a lot of diapers. I think the most important thing about my trip, though, is I went in with a list, and 7 coupons, and I walked out with ONLY the things on my list. So, that's what I do when I need something.