Have a budget, decide how much you and your partner are able to spend on the wedding, how much others will be giving you (your parents, his parents, other relatives and friends etc). Now do some initial research about the cost of various elements of the wedding. Have a rough cost of the reception per head, invitations, flowers, various types of gowns, transport, rings, etc...Now go back to your budget and work out what fits and what needs to be trimmed a bit. Before going out and falling in love with a wonderfully expensive gown, decide which proportion of this budget you would like to spend on a gown, and book shops/designers accordingly (for Couture Gowns, I have noticed that the average spent on a gown is 10% (approx) of the overall budget of the wedding, just as a point of interest!)
Most preferably have a firm date booked, or at the very least a specific time of year for the wedding...different seasons mean you will want to or have to choose differently from what you may have in another season.
Have preferably booked a venue for your service and reception, or at least know if you will be using a church, or outdoor space etc.
Do some research and put together a "Look Book" for your wedding...Gowns, Colours, Flowers, Decorations etc...this all helps wonderfully for the designers to "get inside your mind" and help make your dream a reality.
Make a list of designers or labels that have Gowns you like the look and feel of. It is better to keep the list to about 3 to 6 designers/shops. Any more than 6 can become confusing, not to mention exhausting.
Keep a very open Mind and plan to have a wonderful time, plan stops for lunch and coffee, or drinks afterwards.....what other time will you ever get the chance to play dress-ups with so may stunning (and expensive) dresses.