I've played around with t-shirt yarn a little bit ... you don't even need to spin them if you don't want to. If you tug the cut strips, it will curl into a tube on its own. And T-shirts are perfect, as the body is a cylinder, so you can cut a continuous length :-) ... that said, plying with thread or something would let you join shorter lengths into one mondo skein, or switch t-shirts from time to time for a multi-colored yarn. Hmmm...
Right now I'm totally enthralled with my Majacraft Suzie Alpaca wheel ... right down to the cute little alpaca logo. It does everything I need, and I'm loving it. If I had room, I'd probably spring for a Reeves-Schacht or Lendrum Production wheel; the accelerator head on the Majacraft gives me speed, but takes a harder treadling than the full size production wheels.
There must be many ways to do this -- love the clothespin idea!
I usually fold over the last 4-6" of spun up yarn so it twirls on itself, then wrap it around the spindle shaft below the whorl. The rest of the yarn on the spindle behaves, and that final section doesn't lose any twist. I unfold it, re-join onto my fiber (if I detached it from the fiber), and continue spinning.
The good thing is, you can thin out the thicker section too :-) so it matches the rest. twist generally wants to be in the thinner sections of fiber. Get some more twist into that section once you've thinned it out -- wind it into a ball up to that section, stick the ball on your spindle and add twist to that spot, then finish winding it into a ball. That's how I'd do it, with a spindle at hand. Then I'd skein it up and wash it again, so the twist can migrate through that spot in the skein and average out with the rest of the yarn.
A larger spindle will also have more momentum, and keep going a bit longer, perhaps. How large is the one you are using for this now (weight/whorl diameter)?
I roll my spindle on my leg, right hand rolling toward the knee with the spindle, for a counterclockwise rotation -- that gets it going faster than a finger-flick can.
Also, I'll prepare the singles by making a 2-strand ball, so I'm not fiddling with them during the plying process at all -- speeds it up greatly, and winding the ball is fairly quick as well, so I figure it's a net time-saver.
I recently ran across a *really great* thread plying technique on a Patsy Zawitowski video -- put the spool of thread in a sandwich baggie (ziplock or not, as you leave the end open), feeding the end of the thread out the open end of the baggie. Take a large-ish rubber band and rubber band the baggie to the underside of your wrist, feeding the thread out of the end of the baggie. The rubber band shouldn't be super-tight, so the thread can come out.
I've done this with spools and with little sewing machine bobbins -- the thread stays under control, nearby, and feeds out as you need it. Just remember to take the baggy off when you get up from your wheel