Consider yourself lucky if the plane takes off and you have an empty seat next to you, and no strangers to deal with. Travel Codex offers a few strategies to increase your chances of getting lucky.
Even if there are many empty spots available when you select your seats, it's possible the plane will still fill up by the time of departure. You'll have a better chance of having no one sit next to you if you choose a seat in the back (particularly an aisle seat, since many people dislike climbing over strangers on the plane):
[Many customers don't pick seats ahead of time.] If those passengers wait until they check-in, the gate agent might select a seat for them. The gate agent will make some effort to space out passengers, but when things get full they generally move from front to back. If you are worried about a flight selling out and losing the empty seat next to you, sit in the back because that seat will stay empty longer than any of the others.
The article also recommends looking for a cabin configuration, if possible, of 2-3-2 or 2-4-2 seats rather than 3-3-3, because there are fewer middle seats per row. If you don't have any options except that centre section, choose a spot that leaves only one empty seat next to you rather than a lot of empty seats, since people will be more likely to try to sit elsewhere.
How to Pick a Seat in Coach for International Travel [Travel Codex]