For you who find it difficult to center in the thermal, try this:
Oce you read a positive climb on the vario, fly straight ahead for roughly 3 seconds. Then turn into the wing that lifted when you entered the thermal or initiate the turn in the direction where you think you have the most part of the thermal. Chances are that you entered a bit offset from center and this will cause the wing closest to the center to rise. Make a complete 360. Note when the ROC is the worst. At these points, look into the wing and take out a landmark on your wingtop position. When passing this landmark in the turn, go straight for roughly 3 seconds and start turning again. This will after a turn or two center you in the thermal and you'll maximise your ROC.
That's the scientific and technical way to do it. To really maximise your flying, rely on your gut feeling if you have one...
/lns