Wear good underwear - they literally stripped me down in the first shop I went into
Agree with other posters - get a friend/mum/assistant to randomly pick out some for you - they look totally different on you than on the hangar
Experiment with material. I was determined to have a big fairy princess like dress with lots of floaty netting. Made me look like a fat christmas fairy. Structured satin was much more flattering on me
Make appointments and stick to them. Dress shop assistants don't mind a bit of unplanned browsing but if you start to make noises about trying a couple on they will go mental as they cannot cope with the pressure. Really they want to give you their undivided attention.
Try the dodgy-looking shops at the end of the high street next to the charity shops. They look naff from the outside but many of my friends have found their beautiful dresses in them.
Schedule in lots of tea and cake stops. It's knackering.
When your mum says "well if YOU like it, then I'm happy" she hates it and it's likely to be the wrong choice. When your mum starts dressing you up like a Barbie in veils, tiaras, shoes, gloves, it's a likely indication she thinks you look amazing and it could be a viable option.
Haggle over prices. We got my dress, veil and tiara from the same shop and they ended up giving us the £150 tiara for free.
I loved every minute of my dress shopping and took 4 months to find mine so most of all, enjoy it, you'll only get to try them all on once (fingers crossed 
)
Have fun!!