I was there last month and someone from the west end board suggested this site http://theatermonkey.com/ I used it to buy tickets to Les Miserables and Dreamboats and Petticoats. There's some great deals if you have the time to look around the site. I also went to TKTS booth for One Man Two Guvenors. During the week there is practically no line.
It's worth checking out obstructed view seats. Sometimes they're front couple of rows so you can't quite see feet once in a while but they are great seats otherwise. Great prices too.
Also many West End theatres do day seats - ten to twenty-five pounds, depending. I just saw The Bodyguard that way. The seats are usually in the first row and in most theatres that's fine. Turn up at the box office before it opens at ten.
I believe the Donmar now holds all of the front row stalls seats for same-day purchase, but I don't know if they are discounted.
Lots of London theatres have cheap day seats.
The Royal Court has all seats priced at 10 GBP on Mondays. All seats for "The River" are day seats, available at 9 a.m. online or 10 a.m. at the box office. (But the online seats can disappear in a matter of seconds.)
The Almeida has good prices on obstructed view seats, and I understand the views are still pretty good, with just narrow pillars in the way.
Sometimes the National has seats for 10 GBP, available in advance.