If you have a smart phone (specifically android phone), there's an app called "Math Workout" and it let's you choose timed set of 10, 20, or 50 problems. There are games where you have to race against the clock and there's the old math blaster game.
I don't make the app, so I'm not really promoting this particular one, but as I was experimenting, my quant speed has gone up dramatically after playing for a couple of days (nerded out on it though).
I'm not the biggest math buff so sometimes when I'm solving GMAT problems I'll get stuck on an easy problem such as 23-7=???. That's not the kind of problem you want to spend 30 seconds on writing out.
If you have a smart phone, just type in "math" to the marketplace
or app store and you will probably find something similar. I play this at work, when I'm waiting at offices bored, and sometimes I'll get in a few sessions before lunch. It's purely a supplemental tool that's FUN. Because we all know studying multiplication tables is awfully boring.
Try it out and let me know if it helps