The consulation is every bit as important as the event, and for a wedding, especially so. The first thing is to determine what the bride and groom would like as an end-product. Ensure that there is a clear understanding between you and them as to what is expected and what will be delivered. This must also be VERY clearly spelled out in your contract.
Ensure that you know exactly when and where you are expected to be, and for how long. Ask for a list of 'must have' shots and ensure (in the case of people) you know who the subjects are. If this is a church wedding, you will also have to meet with the deacon/pastor to discuss what is and is not allowed within the church before, during and after the ceremony.
Discuss locations for the 'formals' if desired and as well as an alternate (Murphy's Law is always at work). Ensure timings are reasonable. Brides & grooms often think that formals for 20 can be completed in fifteen minutes. I generally give them a time estimate of twice what I think I will need hoping to get at least half of what I want.
If possible, have them bring the Maid/Matron of Honour or get her contact info as well as that of the wedding planner if there is one. You'll need help from both of these people to locate family members, "herd the cats" and so forth.
A few tips: Vist each venue ahead of time, at about the time you'll be there to shoot to get an idea of the light and conditions. Note parking locations and ensure you have sufficient change for meters/parking fees. Know how long it will take to get from 'A' to 'B' to 'C' etc. Make sure you have a list of cell #s for all the key players.
MOST IMPORANTLY: Reveiw the contract, get it signed and GET THEIR DEPOSIT!
Good luck!