fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the
headwind during each turn (kind of like
turns around a point in power flying).

Hmm! Don't understand this and haven't ever done it.

The best way to explain this is to show an extreme example.
Assume a constant wind from the North at 10 knots, and
a stationary source thermal. Also assume that the lift in the
thermal is exactly the same at every altitude, and
the thermal has constant diameter.

The thermal is now a column that tilts south as it rises,
but the column never moves, rather like a tall leaning
tower of Piza (sp?). It remains fixed relative to the
ground.

Now assume that a perfectly centered glider in the thermal
has just enough lift to remain at a given altitude.

If the glider keeps exactly the same bank angle and pitch and
rudder on every 360, the glider will drift with the
wind and exit downwind of the thermal and begin to sink.

So the pilot should extend the upwind time and decrease the
downwind time, to fly a perfect ground reference circle and
remain in the thermal. The ASEL practical test asks pilots to
shallow the bank into the wind and steepen it on
downwind to accomplish this.

I'd guess a lot of competition pilots probably don't
consciously think of this, as their actions to center a thermal
are so subtle that changes in airspeed and direction with
altitude override the primitive assumptions presented here.

Mark Boyd