If you are flying BA, they have a toddler seat that clips into the bassinet area for children up to 18 months (bit like a baby bjorn rocker) so that would be free like the cot for smaller children.
We have not booked an extra seat for our 21 month old for our upcoming long distance trip, but are calling the airline each week to see how full the flight is getting - if it looks like being full, then we will suck the expense up and book a seat for her.
I travelled to NZ with her when she was 14 months without a seat and it was NOT FUN... they did let us squash her into the cot (legs hanging over the end!) but no-one was very comfortable really
Definitely the DVD player as those above have said, that was a life saver. Playing in the galley also - if the flight is not busy, the staff are usually lovely with toddlers (it's in their interest to keep them happy and not annoying other passengers!)
We've also used phenergan and actifed (as prescribed by her Ped) with varying results... e.g. if they are already upset, then don't bother - you get the opposite hyperactive effect!
Good luck