National Theatre - Entrypass
Entry Pass is a free membership scheme for anyone aged 16 – 25 that enables you to receive £5 tickets to all NT productions. If you are over 25 and a show is not sold out, they also offer a student standby rate of £15 in person from 45 minutes before the start of a performance.
Old Vic - PwC £10 Tickets
£10 tickets for previews of all their shows. Sign up for their mailing list to get alerts when they go on sale to make sure you can nab them early.
If you're 16-25 or a student, sign up for free and get access to £5 theatre tickets for RSC shows in Stratford-upon-Avon and London. Attend five shows and you'll a ticket for your sixth show for free.
The Young Vic offers a number of £10 tickets for every production for full time students and people aged 25 and under. No sign up is required - just select 'concession' when you buy and bring proof of your student status when you pickup your tickets.
For something off the beaten track check out Ovalhouse in south London for off-West End and fringe productions. Get tickets to main house shows for £7, concessionary tickets for the downstairs venue from £3 and, bottles of Becks for just £2 with student ID.