When you hired the serving staff, you should have given them the date, general arrival time, party address, and your phone number(s). If they are a direct hire you should have their cell phone numbers as well. If they are through an agency, you should have a contact number or two for any "party day" issues.
You may want to call within the week before the party to confirm staffing arrangements through both the agency and direct hires.
Everyone doesn't need to come at the same time! 2 people, the food/kitchen person and another who will can help set up chairs, the buffet, etc... should arrive 2 1/2 hours before guests, and 2 people should arrive 1 hour to 45 minutes before guests get there.
Let the serving staff know what the dress code is: Nice white shirt, nice black pants, black shoes, black bow tie - or whatever you have in mind.
Make sure that your "communication" chain is clear. Who do they go to for any issues? Typically a staffing agency will assign a "manager" or person that signs every one in and out.
Create a party timetable if necessary and post it in the "working kitchen" area or distribute it to all who will be "running" the party so you can sync up on when the buffet opens or planned events, like serving a birthday cake.
Expect each staff member to take a meal break. And, make any "etiquette requests" or party protocol clear such as "Please don't eat food in front of the guests or behind the buffet." You may want to request that they refrain from drinking alcohol while they are working.
I always touch base with the bartender and let them know what my feelings are about guests that seem to be chugging down the cocktails. Ultimately, it's a party host's responsibility to keep an eye on this and find a ride for a person who has had too much to drink.
The serving staff should leave their "kitchen workspace" in your home in the same condition as they found it. Don't expect them to wash your windows, but they should sweep the workspace floor or the place where they've prepared the food. Plates, glasses, and trash should be picked up in the party space and taken to your trash bins outside, but don't expect them to clean the rooms from top to bottom!
If you did get rentals, find out how these items need to be situated for pick up. Sometimes the staff will help get these in order for you, but it's not necessarily their responsibility.
For direct hires, it is customary to pay direct at the end of the night or when you let each person go home. In LA and New York staff earns around $20.00 an hour, give or take a couple of bucks depending on what they do. I wouldn't go below $10.00 as waiters are used to an hourly wage plus tips. Gratuity on top of the hourly wage should be applied if you are happy at the evening's end.
Staffing agency rates usually run higher as they have more overhead, insurance, and more to account for.
Finally, treat the serving staff courteously and your party should be a success. A verbal "thank-you" is always appreciated!