Tie your baby to a chair! (I did.)
Babygadget mentioned the Sack 'n Seat today -- a fabric doohickey that converts a standard chair into a baby-safe highchair stand-in. Nice for babies who can sit up but are still being hand-fed. Not as useful for toddlers who feed themselves, as they won't be sitting at the proper table height.
When my son was little, in a pinch, I tied him to a chair with the arms of my jacket and then scooted close enough in my own chair to stabilize him with my knees.