Cool your brownies completely, then put them in the fridge. When you finally get to cutting them, use a flat (not serrated) knife and push down in one motion. Pull it out in one motion as well. Clean the knife after every cut. However, plastic knives are rumored to cut brownies better. I've never used a plastic knife, but the Internet says brownies don't stick to plastic knives in the same way they do to metal knives.