Hmmmm as far as Spanx go they have different levels of "Suck it in". Try on different styles and pick one you feel comfortable in. Also ask a consultant. Tell her you're trying on wedding gowns and what your areas of concern are and she'll help guide you.
Definitely wait on any accessories. Go ahead and try on a veil, belt, hairpiece, whatever at the salon but just take notes. You can find all of that for significantly less money off Etsy or from a second hand bridal shop.
Be strict about your budget. Most good salons will work with you but there are others out there who will constantly try to upsell you or push you out of your comfort zone with money.
If a salon will let you take pictures then definitely do it! It was nice to be able to compare how I photographed in a dress to how I looked in the mirror, plus it helped me remember the dress and compare it to others.
Don't book more than three appointments in a day and try to take coffee or lunch breaks in between. Make sure to leave enough time between appointments for travel, traffic, and appointments that run over schedule.
Call me crazy, but I also think that shopping this early could come back to bite you. Your wedding isn't until December 2016, right? You are essentially TWO YEARS out. The styles of dresses will completely change between now & a year out of your date. YOUR style could completely change between now & a year out of your date. I'll be the one to say that I think you should wait.
My wedding is in 6ish months, and I haven't gone shopping dress shopping yet.... hides head in shame.
As punishment we expect the ultimate AW dress shopping post when you finally DO go.
@Blackbird230 Just jumping on the train, I forgot your wedding wasn't til the end of 2016. I'd definitely wait to do SERIOUS dress shopping until you're no more than 12 months out. Styles could change significantly and so could your personal style. I'd just hate to see you get stuck with a dress that you fall out of love with. It's still cool to go shopping now, but don't shop to buy. Wait on that until later.