For a short (under 1000 miles) domestic flight, I generally don't care so much about the food - I usually look at how booked Y is and whether or not I think someone will actually be sitting in 21E (I'm almost always in 21D, and I'm almost always on an MD-80).

If the flight is going into or out of ORD, I typically grab the upgrade because I know there is a good chance you'll get "bonus time" with your upgrade. Last time I upgraded my 400-mile MCI-ORD flight, I was three beers deep before we even took off.