Probably a bit late, but I always attach lines to the end of the bladder before I take it out, then use these as a draw to pull the bladder back in. Ive never had a twisted bladder yet, apart the time the pub dunnies were closed.

Use the coathanger and tape to pull some lines through, then use the lines to do the above. Watch that any sharp ends on the coat dont snag the dacron.

This is the best method I have used and worked first time for me - no powder required...

Lay the leading edge flat on the ground with the opening facing upwards (you'll be seeing the side that's normally against the canopy). The canopy will probably be bunched up a bit to allow this. Just make sure that the leading edge is not twisted.

If you have not done so already, feed a long piece of string (kite line works well) through the leading edge.

Here's the part that makes the difference. Lay the bladder out flat with the inflation point facing up. Starting from one end start folding the bladder up in a zig zag fasion back and forth (i.e. not rolling it up) until you get to the middle. This makes sure that the bladder stays flat. Then do the other side.

Carefully place the folded up bladder next to the opening and attach the kite line to the end of the bladder. Slowly pull it through the leading edge and watch the bladder unravel.

Do the other side. Close off the ends.

Zip up the leading edge and pump it up about 70% to make sure that its not twisted. Then pump it up all the way.

Hope this helps. This worked for me first time round.

I second this method, works heaps better, still got to have someone else to make sure the bladder doesnt twist as it enters the kite. A ruler would do the trick as well if you arent an ex windsurfer

watch the sharp edges of the zipper!! Have had a few things caught
in zippers over the years

Stake the end of the kite into the ground with a screwdriver, and place another stake further down to act as a pulley for your string. That way you can feed the bladder into the zip with one hand and pull the bladder string with the other hand from the middle and you don't need two people. Note which end goes in up on the end of the bladder and if it doesnt come out up at the end of the kite, pull it out and do it again (only takes a minute). It will rarely rotate more than 180 to give you a false confirmation.