When I thermal, there generally seems to be wind,
and since the glider doesn't climb quite as
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).

I started using slips in thermals after I flew
with a competition pilot and noticed he did this
a little when thermalling in steeper banks. I
also sometimes slip a little "into" a thermal.
I suspect this happens mostly because I'm
getting fatigued and my coordination is
getting worse and I'd rather slip into
it than skid. But there may be some
aerodynamic reason for slipping a little.
It seems that at the extreme (in a 90 degree bank),
a slip is better than coordinated flight since it
exposes more fusealage area to the thermal when
C.G. is forward.

There is also some coriolis(SP?) effect, so I notice
on .igc traces of extended thermalling that the
thermal circles a little as it rises.

I also fly by the rule that the center third
altitudes of a thermal often provide the best lift.
Sometimes I fly in the upper third if I expect
to cross a sink area, but the middle third
has been pretty good for me.

I definitely trim for the thermal, and I've
never had a consistent thermal greater than 8 knots,
and I haven't found constant banks greater than 50
degrees useful.

Some of this comes from discussions with Serge Serfaty,
a fellow glider pilot. But he wasn't a big fan of
any uncoordinated flight :P

I've also had to use shallower banks or leave thermals
because I was getting dizzy or tired or hot or couldn't
track a fellow glider. I think these are other
factors that vary based on the pilot.