I agree with PP who have said that it's easier to travel when they're small. We took DD on her first flight at 4mths and she was great - slept for most of it, and pretty happy for the rest.
She was on formula (not sure if you are formula or breast feeding), and we had a bottle warmed ready for takeoff - but then they took so long to take off that I ended up giving it to her early. It wasn't a problem, and she didn't seem to notice takeoff at all!
I'd recommend packing a couple of 'single use' nappy bags, each containing a nappy, nappy bag, and wipes. That way you can throw the whole thing away if you have to do a nappy change on the plane, instead of carrying a bigger bag + baby down the aisle.
We've flown with her several times now, and I've found that most airlines are really helpful if you have a baby - and you also get to board the plane first for some!
I wouldn't use any drugs (someone mentioned fenurgan) - aside from what Witwicky said about SIDS, there is a % of kids for whom fenurgan has the OPPOSITE effect, and it makes them go a bit over the top. Not worth any of the risks, I would say. Plus most people I know who've flown with babies (me included) have found it trouble-free without needing to unnecessarily medicate.