I flew with my son alot before his 3rd birthday (then we moved closer to family). The things that worked for me and him...
Always took his carseat and he had his own seat. Yes, it's more expensive, but it was best for us. He was used to his carseat from car rides and fell asleep easily when strapped into it. Besides, who wants to hold a 18lb baby for 4-5 hours? :-) If you hold the kid you can't put your tray down to eat or get a break to read or relax or whatever while he's sleeping.
If you are nervous or anxious, he will be that way, too and much more likely to cry and fuss.
We always flew during the day, leaving around lunchtime. By 6 months he was on a pretty predictable food/nap schedule so this made it easy. We'd board the plane and I'd feed him (yogurt, applesauce, etc) while everyone else got on board. When the plane got to the runway, I'd put everything away, lean back and turn my head away from him. Basically ignore him and pretend to sleep... this was his cue (along with full tummy and normal nap time) that it was time to sleep. 99% of the time, the rumbling of take-off would send him to sleep and he'd be out for the first 2 hours of the flight.
I try flying at night before but it wasn't his normal schedule (he was awake later than normal bedtime) and he was very over-stimulated on the plane so he just cried the whole time and never settled down. I also tried holding him once on a 1 hour flight - disaster. He was so squirmy and not used to falling asleep in my arms while traveling so he was awake the entire time (even though he was overtired and cranky).
He will probably cry. Most people know this. I had very few negative experiences with other passengers. When he cries (not if!), do your best to get him to stop crying... food, toys, crinkly cup, change the diaper, walk him around plane, etc. (but if he's tired, don't overstimulate, may be best to wrap up tight in blanket and jiggle/bounce/walk to get him to nod off). Most people are nice and understanding as long as they can see you are trying your best to get him to stop.
Most important - keep your cool. Babies pick up how they should react from parents. If you are anxious and nervous then he will think he needs to be to (after all, there's a reason you are acting that way, right?).