I was out of school for five years before grad school, and I did exactly what JSE recommended and I highly recommend it: I sat in on a graduate course at a (well-known) local university, to make sure I wanted to really quit my job to go to grad school, to give me some idea of what I might field I wanted to go into, and to make sure I had the mathematical chops to have a reasonable chance at success. My professor wrote a rec letter for me (which surely helped) and also gave me very useful advice on applying to grad school. All of this made a huge difference.