In addition to the points boeingbus2002's very good list I would add:
Visit a few schools and look at the facilities and chat to current students
Find out what their pass rate is like from previous/current students
If the school has a mix of integrated and modular - find out if you may get 'bumped off' in favour of an integrated student.
Test centre location is quite important - some like Oxford, Cranfield & Bournemouth complex departures & arrivals as well as possiblities of getting a test at places with 'interesting missed approach procedures' like Coventry.
See if the school has something like an evaluation offer. I seem to remember seeing that being offered by a couple of schools. If they have take them up and compare.
Don't take the marketing/sales hype too seriously - eg the 'Our School on your CV will look good to a prospective employer,' rubbish.
If you know someone in the industry who can give you some unbaised advice, eg BALPA or IPA representatives would be a good bet.
Above all take you time and make your choice carefully, after all think about the time and consideration you would take to make a capital purchase like a car or a house - you will be forking out a not too dissimilar piece of your personal equity for the course!