I fly Coach all the time, except when I get upgraded with AA's "stickers".

You are right, it makes a world of difference when you have an empty seat next to you. Here's what I do:

-If possible, try to fly Tues, Wed, Thurs.
-Select an aisle seat in the middle section towards the back of the plane. The "Type A" personalities are towards the front.
-Watch the seat map before your flight, and quickly switch if some rotten person takes that seat next to you.
-Avoid selecting seats in two-across sections. All seats in two-across sections are either aisle or window, and they get filled before middle seats in the middle 3-across section.
-If you really want to get weird about it, get the numbers of a few flights you are considering, then look at "today's" flights to see which has the most empty seats. Do the crowds go for early morning flights? If so, you go for a later flight.
-If you see an opening (empty aisle seat with empty middle next to it), plop your derriere in it just before the doors close. If you wait until after the doors close, you may lose it to someone quicker on the draw. If you move too soon, you might have to slink back when the rightful owner shows up.

Finally, if you are flying Coach, learn to love it. Take your own IFE and zone out. An audiobook on my iPod keeps me entertained for hours. I check for someone behind me, first, then tip my seat back a bit (all the way if no one behind me), plug in the iPod, close my eyes and just veg my way across the sky.