My first flight was as a 6 week old from LHR to Tripoli, Libya, but the first one I can actually remember was LHR to Basle on a Swissair Metropolitan (hence the avatar).
Interestingly, the enduring memory is not of the flight itself, but rather the catastophic event that took place on take off. The Stewardess had given me a small bar of (naturally) swiss chocolate which I carefully placed on the small shelf in front of me, just out of my reach once I was firmly belted into my seat.
To a four year old boy, the excitement of the take off was no match for the anticipated delights of that chocolate bar, and my gaze didn't leave that small block of pleasure despite the scenery that flashed past the square windows of the Convair 240 as it rushed down the runway.
As we started to climb, the chocolate bar started to move towards the edge of the shelf, gradually increasing velocity, until, to my abject horror, it slid off the shelf, past my straining stretched out arms and onto the floor. Then, as the aircarft rose steeper, it elegantly slid under my seat, and probably several rows more, leaving me with my mouth and eyes wide open, the former emitting a banshee like wail and the latter rapidly welling with up with tears.
My Mother thought my calcophony was caused by the air pressure, and it wasn't until the stewadess brought me another chocolate bar that I was able to coherently relay to my parents the real reason for my distress.
I remember that event like it happened yesterday. It's a pity I cant store my PIN numbers in the same part of my brain.