They ran this promotion last year, and managed to go to a few good shows with it.
I know the getintolondontheatre.com promotion has already gone up, but this is one specifically for 16-25 year olds. Most of the seat allocations are pretty good.
I went to see Wicked the other day using this, and sat at the back in the stalls - amazing view, and for £15! Seats here are usually £60
Offer runs up to the 8th Feb