Here are a couple of tips for making a great cut for bars.

When you cut brownies, it always seems as if the brownies want to come up a bit with your knife.

The best way to make sure this doesn’t happen is to let the brownies cool thoroughly before cutting them.

But who doesn’t love a warm from the oven brownie?

So, another option is to layer your baking pan with aluminum foil.
Let the ends hang over.
When the brownies are done, you can grab the ends of the foil and lift the brownies out of the pan.
Turn the brownies upside down and remove the foil.
Cut the brownies from the bottom side.
You will still want the brownies to cool at least slightly before cutting.

Another great way to cut bars is to use a pastry scraper to make cuts. Just push the scraper straight down and you will get an even cut. I have never ended up with bad edges cutting bars this way.