You will need a fence or wall 2-3 feet taller than your player. Stand sideways next to the fence/wall with your non-dominant foot touching the fence/wall and your tossing arm closest to fence/wall. Toss ball as if you were going to hit an overhand serve, so ball needs to be 2-3 feet higher than your head, or about as high as player can reach with racquet. Trap ball against fence with racquet.
Players will soon find this is easier to do with a good quality toss out in front of them (which hopefully they have perfected by doing the Tap N Toss activity). Tossing behind them pretty much makes this impossible.
Once player is comfortable with this, have them turn 180 degrees and face the net, or move to the actual baseline. Visualizing the fence trap activity (now imagining the fence at the baseline), have them try tossing and 'trapping' the ball. Since there is no fence/wall in their way now, they should have a pretty good quality point of contact at the baseline.
Notes: if you are using a fence, avoid the support poles. Very hard on the racquet.
Here's a video of one of my Red Ball students performing the activity:
Also here's a great article from about.com listing some other common-sense strategies for improving the serve.