I live out of London and usually obtain an official West End shows leaflet from our local TIC. Once I have decided which show I wish to see I phone the Box Office direct - the phone numbers are given in this leaflet - I never deal with ticket agencies. They give the best prices I have found and if you can give as much notice as possible you should get good seats. I have found however that phoning just a few days or even less before I wish to go and if I only want one or two seats I have always been lucky. They may not have been top price seats but I normally pay only around £30-40 per seat and it is normally a pretty good one. Some theatres also have a seating plan that you can download to help when talking on the phone. Booking direct with the theatre's own web site also gives better results than a ticket agency. Be aware that you might have to pay a fee on top of the ticket price however you book.