The most important advice for anyone paying for a service of any kind: get the details and get it in writing. While a verbal agreement may seem as good as a handshake between you and your vendor, i've seen far too many cases where vendors have taken advantage of clients in the worst way. Never assume that your nice "mom's friend from work" will honour her word, and it is in your relationship's best interest that you put it in writing, to let both parties know exactly what is expected. Here are some examples of details you might want to include:
Photographer- Hours of coverage, number of assistants provided, any additional transportation costs. Cost and exact number of prints received, specifications of any albums he/she is including in the package. Are CD's of digital copies included? If so, are you allowed to reprint those (some photographers will only allow you to reprint with their permission).
Florists- All flowers, rentals (vases, linens, chairs, draping, lighting, etc.) they're responsible for. Set up and clean up fees, transportation fees, additional labour fees. Does it include samples of bouquets and/or centerpieces? Is there a site inspection included? When are rentals expected back?
Musicians/Entertainment- Time, date. How many musicians, instruments are included? If you booked a particular talent based on a demo (ie: singer), make sure this singer's name is included on the contract. Additional travel time. Also detail how much it is per hour if they run overtime.
Finally, have copies of all your vendor contracts and leave them with the General Manager. Make sure you indicate all the payment details, leaving checks or cash with someone responsible. The last thing you should be thinking about is if you've paid off the DJ!