Ways to thicken acrylics?

I have recently gotten into painting and was wondering if there were any thickening agencies for acrylic paint. Because I am not a very experienced painter, I don't want to buy expensive oil paints (plus, I have no idea how to handle them:) or how to clean the brushes). I tried to use flour to thicken my paint once, and although it sort of worked, it ruined my brushes and created a very flaky and crusty texture to the finished product. Any suggestions?