Return deeper. If you are used to the receiving opponent coming in behind his serve, you will be used to returning shorter and a little angled. For singles, unless your opponent S&V, return deeper. Try even returning (or blocking the ball) right back at the opponent - his footwork might not be good enough to get out of his own way.
If you come to the net to S&V, chip-and-charge, or otherwise come in behind a shot your first volley should be down the line so you will be able to get into position to cover the greatest percentage of possible returns (though you need to know you cannot cover all).
If you are at all facile about going to the net, you will have an advantage. Try it and see if your opponent can consistently hit good passing shots. Like the previous poster stated - every ball is yours. Don't let yourself start thinking, "My partner would have gotten that if this had been doubles".
From the baseline - keep the ball crosscourt and be very conscious of recovering to a position for the next ball. You do not have a partner to follow in tandem, so the recovery is actually easier in some ways.
Early on in the match, test out what your opponent can and cannot do in terms of shots, movement, and mental focus. If both opponents in singles have decent shots technically, the rest becomes a test of smarts and wills where you figure out how to try to get your opponent to have to hit shots he doesn't like to hit.
Mostly - have fun! I play almost exclusively singles and it is very rewarding to play a good singles match. Good luck.
