Getting a flat tum - any advice?

[xanadu324]

Hands down, an exercise mat and some good old fashioned crunches. Sit-ups tend to strain the neck. There are many different ways to do crunches, so it doesn't feel like such a bore. You could try getting an exercise ball, or search google for different ways of changing up the crunches to make sure you're working all the right muscles.
I hope this helps.