My advice begins with: don't try to save a few dollars by finding some college kids through Craigslist to work as your valets. Hire an established service. One with a verifiable track record.
And don't get your cousin's friend's son to move the cars either. You need an insured business with experienced workers. Far too many things can go wrong here. You need people that know what they're doing.
How long has XYZ Valet Service been in business?
Where were your last 5 house parties?
This goes back how far? (Did you serve them in the last 2 weeks, the last 3 weeks?)
I'm going to need contact info for those last 5 customers.
How many house parties have you done in the past year?
Who are your valets? I need to know their name, age, and amount of time working for you. I'm going to need to see photos of them.
Can they all handle manual transmission cars?
How much do you pay them?
Do they expect tips?
How do you go about safeguarding keys?
Will a manager be on-site to oversee the operation?
Will the manager verify that all cars are secure, with windows shut and doors locked?
Do you check vehicles for damage upon receiving them? Is this information documented?
Who is your insurance provider? Describe your policy. What is your deductible?
What happens if a car gets damaged while it is in your care? How do you handle that?
When was your last damage incident? Tell me what happened.
How often do vehicles in your care get damaged?
Are your workers covered by workers compensation insurance? I will need to verify this coverage. (If workers are independent contractors, this is a red flag.)
I would like to see your training manual. Do you have one?
How do you train new workers? How long does the training last?
How do you screen your new hires? What kind of background checks are involved?
I am expecting 120 guests. The party start time is 6:00pm. We will wrap up not later than 2:00am. How many valet workers will staff my house party?
Can I see the signage and traffic cones you will use?
Where do you plan on parking the cars?
How will you park them?
What will you do to keep the vehicles secure?
What will you do if it is raining?
What will be the charge if we require additional time beyond the expected end to our party?
What happens if a car won't start? How do you handle that?
These are my expectations:
No begging for tips. I do not want the experience my guests enjoyed at my house party to be degraded by lousy attitudes because a tip was not given by somebody.
No coming in my house to use the toilet or to mooch for food. I don't want to see any of your workers in my house.
No smoking in the cars. Or near them. I don't want a single whiff of cigarette smoke in any of the cars.
I expect professional attire.
I expect zero requests for autographs or selfies with my guests.
There will be no pick-up lines attempted. No requests for phone numbers. No love notes left in cars. This is strictly business, not your worker's social time. I expect professional conduct.
There will be no selfie shots with the cars.
I require absolute confidentiality. Everything here, everything with every guest, and who showed up, is strictly confidential. There will be no talk about the event. There will be no social media postings about the event by you or any of your workers.
This type of interview might scare away some service providers, but you are looking to place someone in command of high value property, and it's serious stuff. All sorts of things can go wrong, and you want to have the best chance of nothing going wrong. You do this by screening well, being tough, and making your expectations clear.
If I were hiring a valet service for a house party, there would be no tipping. The workers would be paid a fair hourly rate for their service. Period. And I would provide an additional bonus to the company owner if valet parking for the event went perfectly. This means:
nothing stolen out of the cars
no keys lost
no long delays to have cars returned
no parking tickets
no playing with the car stereos or changing stations
no aggressive driving
I expect cordial, professional service
Now, you might find my approach to be a little harsh, but there is a good reason for it. A lot of schmucks are in this business. Things go wrong all the time when it comes to valet parking. Outsiders say "What's so hard about parking cars?" They just don't fully grasp all of the things that can go awry here.
When hiring a valet service to handle the parking for your house party, you need to do some investigative work to be certain you are hiring a top quality service provider.
Get copies of their insurance policy that protects your guests' cars and you from liability. Call the insurance carrier and verify that the policy is in effect. Become aware of the deductible and press the valet company for confirmation that they have adequate cash reserves to pay the deductible, in the event of a mishap.
Do the same thing with the workers compensation insurance provider. You want to be sure that if a worker gets hurt on the job, they won't be coming after you to pay the hospital bills, etc. (And yes, valet workers do get injured on-the-job.)
Call recent house party customers of their's. See what they say about the valet company's performance. You might even try reaching out to neighbors where those house parties were held to see what their opinion is.
Check Yelp.com for reviews and feedback on this company. Maybe try a Better Business Bureau check too, (though I do have some disdain for this organization).
Are they licensed? (Is a license required?) Verify that this is a legitimate business.
Do a Google search for whatever else you can find out about them.
This is a production. This is a show. You will need to have adequate staffing. Will all guests arrive at the same time? Will they all leave at the same time? This will require extra workers to avoid service delays.
Do you want your guests to have a departure amenity, like cold bottles of water? Now is the time to work out this detail.
There will probably be a contract. Review this carefully. Make changes that are suitable for you.
You are paying a top price for top service. And you expect to get it. So make that clear.
Have a nice party! :)