I have been there and done that, i.e. the travelling with kids thing. My son is 3 and can be a handful on the flight between NRT and SFO. He is good for a while and then gets fed up. I have the following advice to offer.

1. Try a series of decongestants (not all at the same time)with your child several days before flying to see which works best. Benadryl puts some people to sleep and makes others hyper.

Dramamine also works well towards calming kids as well as their stomachs. It comes in kids strength and contains some deongestant I believe.

2. Always travel with a partner. This means that you and your partner can trade off carrying/holding/playing with etc. the child.

3. Talk about the trip as much as possible before hand, maybe even set up a mock seat on the couch for a few days before the flight. Make a game out of it and make the child feel as proud of themself as possible about accomplishing the simple things like fastening the belt. Discuss in depth the fact that the seat back in front is connected to a nother passenger's seat and is therefore off limits. No using it as a brace to hoist themself up to stand up.

4. Whenever possible fly economy with the child and spend the difference on yourself next trip.

Finally, lots of love and strong alcohol at the other end of the trip helps too.