How can I indicate that that a check is valid only if deposited no later than a certain date, say December 20, 2007?

By writing on the check valid only if deposited by 12/20/07

Is that all it takes to do such scottgem? And does that apply in all areas?

I do understand there are some issues, such as that it has to be valid for 90 days I thinik ( this is a I think) but yes beyond that, you put a date it is no longer valid on,

Now with that said, if this is to a large company like the electric company or phone company their checks are not processed by hand, but by machine, and notes on them are not read, so the check would still be proccessed anyway.

Taking into account Chuck's caveat that the check might not be processed by hand so that the disclaimer might not be read, it should still work.

Large companies will likely process checks as soon as received and if a check falls through the cracks, its likely to be read before processing. But even handwriting such an endorsement on the check will give the payor some recourse if the check is delayed.