This can take on many forms, really, but I’ll get your juices flowing with some things we do on a day to day basis (I’m sure you can think of more, and I’d love to hear about them below!).
Cloth diapers. We started when #1 was 8 months old (only because we had no idea how simple it could be beforehand), and have cloth diapered every child since then. It’s really super easy, and as long as you don’t mind doing a little extra laundry, it can literally save you hundreds of dollars. You can get an entire set of diapers for under $200 that will last you from birth to potty training (our favorites, and the ones we’ve used on all 4 of our children exclusively, here—affiliate link).
Clothing. Though it’s nice to wear something different each day, is it really necessary, if the clothing itself isn’t actually dirty? Make it a practice, whenever possible, to wear things 2 or 3 times before washing. That means you’re doing less laundry, and your clothes are getting more use out of them.
Furthermore, buy used. We almost always shop Goodwill (or the equivalent) for clothing (especially kids–they grow so fast, there’s no way I’m spending $15 on a 2T shirt to have them wear it for 6 months!), and Craigslist for toys, appliances, furniture, etc. If you have to buy it from a store, check their scratch-and-dent, as-is, or refurbished section–a lot of times you can get a far better deal for something with very minimal or near nonexistent defects. Just do your research and make sure you’re getting all of what you pay for.