There are two critically important things, in ensuring that your
training flights are not "wasted":

1) Your preparation. Prepare ahead of time. Most clubs are really
not that good at the ground-instruction side of things - so you have
to do a lot of the leg-work on this yourself. Be self-motivated, find
good books and DVDs on transitioning to gliders and learning to fly
gliders. Read / Watch them, and really try to absorb all the
knowledge from them that you can.

2) Your instructor(s). Many clubs have rotating duty cycles or
volunteer instructors that come out approximately "whenever they feel
like it" Their volunteerism is commendable, but it makes for a slower
training system because you will be repeating maneuvers to different
instructors (so that they confirm you know your stuff). Try to work
with just a few instructors - be respectful but aggressive in getting
just one or two scheduled to fly with you as much as possible. In my
club I flew with 6 (!!) instructors in my first few months; as a
result it took 20 flights to go solo (transitioning from PPL SEL) when
it should have taken 10. Once I *was* solo, none of the instructors
wanted to prep me for my ride. So I went to a commercial operation to
finish up and take my checkride, and got through in 3 days and only 7
or 8 flights (including my checkride flights).

Regarding flight length: Its hard to swallow this when you're eager
to soar - but thermalling and long flight times are NOT something to
aim for early on. Getting the instructor in the back seat, running
through the maneuvers (and getting comfortable with them) is the
hardest thing to schedule. More short flights is better than fewer
long ones - even if it costs a few $$ more for those extra tows.
Remember that once you're signed off to go solo, you can practice
thermalling and try to truly soar as much as you want (even before you
get your checkride)... With only a single person in the cockpit,
you'll also find that your glider will climb better and turn better -
so thermalling will be more enjoyable at that point, too!

Finally, one more tip: If you're computer-savvy, there's a great
flight simulator for soaring. Its called "Condor". It is best if you
have the basics of glider controls and aerotowing under your belt
before you try it, but once you're to the point where you want to
practice thermalling, cross-country flight, ridge-soaring, and other
advanced techniques, Condor is the way to go! But again, get
comfortable with gliding flight and towing first.

Take care,

--Noel
P.S. Sorry, there's one more thing some clubs are really bad at:
Teaching new members the non-soaring side of operations! Not only do
these duties help the club, but they help pass the time between
flights and help YOU be a better pilot (no joke!). Go out and have
someone teach you how to wing-run. You'll get to see a ton of
different take-off techniques over the course of a few hours (some of
which you may make a note to try - or avoid!); and you'll learn a lot
about safety on both ends of the line. Also, helping rig and pre-
flight other gliders (club and non-club) can be a great learning
experience. Be respectful of people's equipment and NEVER touch,
unless you're given permission. Seeing different equipment designs,
pre-flight techniques, assembly methods, and talking with other pilots
will help you learn things - some things you won't even realize you've
learned until later! One of the reasons we fly as "clubs" is because
gliding is really an all-day venture - there's a lot of value AND
enjoyment in the camaraderie and in being part of the entire operation
- not just getting "stick-time" and then going home... Have fun!