From a fairly frequent traveller (me): Check where you're seated.
If you're in an aisle seat, or a forward seat, be amongst the later ones to board - it is simply more efficient, and you won't be asked to please stand up while people get in next to you.
If you're seated in the rear of the aircraft, or have a window seat (or middle of a 5-seat block), get in early, to get out of people's way.
Before boarding, figure out what you're going to need (and not need) during the flight, and put things into your carry-on or pockets (or a small plastic bag?) based on whether you plan on using them; don't wait until you're at/in your seat to start sorting through stuff.
The flight attendants can help you with your carry-on (they have more experience than you in this regard), and you don't NEED to have your stuff stored right above you; if you have the stuff you need on your person, there is no need for your carry-on luggage during your flight.
Also, don't explain to "yahoos" that their stuff need to be in specific locations - just move it, and proclaim loudly that people who don't understand the standards set forth by airlines, are idiots :)