Before I went back to school for math (at 35) I literally went to the kid section in the bookstore and bought flashcards for multiplication, division, addition and subtraction. (Funnily enough I found out I was worse at addition and subtraction than multiplication and division.)
There's also stuff online (just google) and also if you have a smartphone there are a lot of great apps. I have a few on my droid that I go through sometimes. I don't want to be one of those people who can only do advanced math but forgets how to do arithmetic, so I intend to keep it up.