I have been traveling since I was little but ever since I started traveling for work I have become more and more intolerant of people and the bad etiquette we all tend to display while on an airplane.  Hence this post/rant:

1. When boarding your next flight try to understand the plane you will be riding on, the smaller the plane the smaller the bag you should try to cram in the overhead.  Don't bring a huge suitcase on a turboprop, it's not going to fit! Instead, ask the gate agent for a gate check tag at least 10 minutes prior to starting boarding, you hold up the agents, passengers and more importantly all of the connecting passengers who are now waiting for you to carry your bag all the way to the front of the airplane to be gate checked because you didn't prepare ahead of time!
2. Be polite to the flight attendants! Most of them are overworked, tired and putting up with crazy people all day.  Saying your thank you's really helps you and them out.  Always say thank you when leaving, it's just the right thing to do!
3. Don't lean your seat back! Planes today are small enough as is and economy seats are tight and cozy.  We don't need to look at the top of your head for the whole flight.  If it's an international flight kindly ask the poor sap behind you if they mind if you lean your seat back before you just plop you head in their lap.  I don't lean my seat back because personally it doesn't make it any more comfortable than sitting straight up.
4. If you make a mess in the lavatory, please for the love of god clean up after yourself! Someone else is going to want to use that toilet on an 8+ hour flight!

Just some of my ranting from my latest round at multiple airports, multiple flights, all jam packed into a 24 hour time period.