E.D.
1. Take a red eye flight. My daughter is/was a very physical infant/toddler (when she started walking it actually meant she would *only* run/climb to her destination. She has always had a "why on earth would you want to keep me still when I can MOVE, mom" attitude and I am lucky if I can keep her still for fifteen minutes) so on long flights to my Dad's, I made sure to book the tickets for when she slept.
2. If your flying during the day, of course pack lots of activities for the baby. Keep them hidden and not played with for weeks before you get on the plane so they are new and exciting when you're in the air. Take one out ever ten-sixty minutes depending on your babes attention span and rotate them so as to keep them fresh. Snacks are great because they also are an activity.
I don't know how you feel about media and infants, but I'll be honest. Had I been able to, I would have had no qualms with setting up an age appropriate movie/show on the I-pod.
Use any and all time in the airport productively...get their energy out! I let my daughter take me on some grand adventures and by the time we boarded, she was ready to nap for a couple hours. Hmmmm, sweet bliss.
3. The ergo, plus my boobs, were of great use to me when my daughter was still an infant and traveling. I made sure to ask for an isle seat so that I could get in and out without having to ask someone to move. Often, I could trade for an isle seat if it wasn't possible to get in advance (mostly, people want the window). I could get her to sleep by walking up and down in the ergo, or just plopping her on my breast.
They will not let you keep your baby strapped to you in the ergo during take off or landing - so if he's sleeping he'll get woken for take off. I curse the time I didn't know about this and had my baby asleep and ready for the trip - only to be woken and grumpy for the next two hours of the plane ride. It did not make for a good start.
4. More importantly, do things to keep yourself fresh. Your babes favorite activity/toy at this age is probably YOU. So, rest before hand and be mentally prepared to be absolutely fascinating for five to eight hours straight. Walks to the bathroom are wonderful. Let him hold onto your hands and walk to get out more energy so that he is more likely to want to sit and play with you for longer. Bring snacks and water (bring an empty container through the security check and fill it at the water fountain once your through security) for yourself (darn those airlines, pretzels do NOT count as a snack!) and get a good nights sleep the night before (if possible). Ask your family to give you a break when you land and know it's coming while your on the plane.
Pack light. Traversing an airport with one thousand bags of luggage is really difficult when holding a babe.
5. I totally hear you about an extra seat. However, I did pay the extra fifty to get extra foot room on my last flight and in was Worth. Every. Penny.
Have a great trip!