Mostly I'm thinking about girls.
What is it that you are really doing?
I think the Red Sox are better than the white Sox
Neither one are going to the playoffs so it's a moot point at best.
I can only report what type of chess-thinking I am doing while playing. My last few games, I came to a point where I was only guessing what to do in a position, instead of knowing what to do.
i'm going out on a crazy limb here and say:
I am thinking about my overall plan at the moment (which does change throughout the game), and specifically my next move and/or my opponent's next move.
in all honesty, (if you are actually trying to win the game) what the heck else are you thinking about if you're not thinking about the above? is there really some psychological aspect that i'm over-looking?
EDIT: and, of course, i'm thinking about my knights facing in the proper direction or else i've already lost...
I also think who is the most attractive girl of legal age in this tournament and if i should take her out to get a few drinks after the game.
Mostly I just try to avoid making 1-2 move blunders, always waiting for my opponent to shatter my make-believe security and dismantle my pride with one move.
Different levels of skill and different styles of player (lets say intuitive vs calculative) will think differently during a game.
If my secondary stuff is sharp and working (checking for forcing moves to win, and checking my candidates to see if they work tactically) then I'm thinking about the strengths and weaknesses of the position, trying to make mine more favorable and my opponent's less favorable. I tend to conceptualize everthing in terms of overall activity. I see activity as directly related to value. Also looking at the area of the board (kingside, center, queenside) each player has play on, and long range prospects like watching the structure and minor pieces in terms of an endgame.
Although the basics are there, fundamentally different positions (say open tactics vs an endgame) will require the same player to think of different specifics... a different monologue so to speak.
Then there are the positions where you're lost, like Trysts said... I wouldn't want to call it guessing... I'd call it, doing the best you can with the knowledge you have 
Beginners, if they were like me, are probably thinking (looking for) traps, hoping their opponent wont see an attack or mate (what I call hoping they get 2 moves in a row) and sometimes checking to see if they're leaving a piece hanging. I can only guess stronger players reference a lot of positions from games they've studied/memorized as well as the standard calculating and evaluating.

Thank you for answering the last topic I posted about your thoughts after a loss. In this post, I am asking for your input on what it is that you are actually doing while playing chess.
Are you sitting there talking to yourself about candidate moves? Is there a runnning internal monologue? Are you managing electro-chemical sensory inputs in order to translate them into a work-able plan on the chessboard? Let me know. I am interested in the psychological aspects of chess.