I have a slow drip on the drain valve on my water heater. We've flushed the water heater about a month ago, but now we have a very slow drip from the drain valve. It's dripping at a rate of about 2 cups a week, no matter how tightly we turn the knob on the valve. Any ideas?

Any ideas? Yes. The valve is called a "boiler drain" and most heaters come with a plastic valve. Yours has some crud caught in the seat. I would replace it with a brass one. Shut the power off to the heater, attach a hose to the boiler drain and drain it down. Unscrew the old boiler drain, Teflon tape the threads on the new valve and removec the old and install the new. You should flush, (not drain) you heater to remove any mineral build up. Let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back togather the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom

If you ant a quick fix, get a hose cap. Hose Cap, Polymer (http://www.lowes.com/lowes/lkn?action=productDetail&productId=219808-306-04HCGF2&lpage=none)

Speedball is 100% correct. The plastic boiler drains are scary at times and they seem to have small openings. I replace them too. There is always the possibility for crud to mess up the seat especially if it's not flushed regularly.