It’s much easier to cut brownies with a plastic knife than a metal one, for some reason. It just is. I don’t know why. But try it if you don’t already do this, and you’ll be amazed at the difference.
Also: if you have to cut a cake at a baby shower/wedding/any event with a cake that it’s not in the pan, take a long piece of dental floss and pull it tight between your hands, and just slice through the cake. It’s easy to see if you’re making a straight line (well, easier than with a knife, at least) and it doesn’t make such a mess. After you cut straight down into the cake, pull the floss out through the bottom – don’t pull it back up through where you just cut. My mother-in-law taught me this trick!
For more of what works, go to Rocks In My Dryer.