@MsGinkgo: Here’s what I would recommend, as a floral designer.
1) Know at least generally what kinds of floral stuff you need. How many bouquets? How many pin-ons (bouonnieres for men, corsages for women)? Any kids participating, and if so, will you want things for them? How many guests do you expect to attend? Will you have round tables, rectangles, or farm-style long tables? Will you want floral stuff for your ceremony area (chairs, aisle, altar, arch)? Other large arrangements for the reception area? Are you interested in floral stuff on other tables besides eating tables, like cake/escort card/gift table? You don’t have to have exact numbers of these things this far out, but most floral designers base their proposals off how many of each thing you need.
2) Have an idea of the look or feel of the event you’re planning. Is it rustic chic? Modern/urban? Traditional church setting and hall reception? The setting and feel for the event can help inform both your color scheme and the flower types.
3) Before the meeting, look at the work that the person you’re meeting with has done in the past, to see if s/he has a similar style to what appeals to you and your partner, florally. I shopw my work to most of my clients when I do an in-person meeting but it helps if they’ve seen some of it ahead of time so they know my style fits with theirs.
4) Are there elements that are really important to you or your partner that need to be included or excluded? Does anyone have an allergy to flowers or a certain flower? Are you really anti-sunflowers? Do you like certain colors or textures or dislike others? Does your culture have any floral traditions that your designer should know about? I work with couples from all kinds of ethnic backgrounds; certain cultures consider some flower types inappropriate for a wedding; others have traditions they like to uphold, like including a sprig of rosemary in a bouquet. Do you have a favorite flower, and if so, are you willing to pay more to have it if it’s out of season?
Bring: pictures of what you/the bridal party will be wearing, if you know it already. An idea of your budget; how high of a priority is floral in your overall budget? Pictures of bouquets or flower types you like or that appeal to you. A pinterest board can be easier than having physical copies of pictures, but if your florist is old fashioned or of an older generation, s/he may not be a big internet user. Swatches of fabric or ribbon or other material, if you have it. Your color scheme, if you’ve figured that out. I ask my clients to share their inspiration boards with me for their weddings in general, not just their floral boards, so I can get an overall idea of what inspires them or what attracts them.
Ask: What is your deposit and payment policy? Cancellation policy? Can you show me recent work? Do you charge separately for delivery and setup? Read their contract, if they have one, to get additional information. I love when clients ask me why I like doing flowers for weddings, or how I got into doing wedding flowers, but not all designers feel the need to have a personal connection with their clients.
Look out for: Someone who seems disorganized or forgetful. Someone who can’t show examples of recent work (either online or in physical photos). Someone who treats you as a cog or is disinterested in your story. Someone who dismisses your ideas out of hand. Someone who is pushy about upselling you or makes you feel ignorant.
Hope this is helpful, and best of luck!