Many factors can make or break a festival or other large outdoor event – the weather, the food, the entertainment. Add to that list the restroom facilities. If portable restrooms are too few or memorably unpleasant, nothing else really matters.

When planning for portable restrooms at your next event, keep these tips in mind.

Don’t skimp on the number

If event planners under-forecast attendance or under-order portable restrooms in order to protect their profit margins, porta potty lines can form, and restroom conditions can deteriorate. At that point, “people start to leave, it starts to affect everything,” said Jerry Vecchierelli, regional product development manager at United Rentals. “Everything” can include sales. “I’ve seen studies where the number of portable restrooms correlates with concession sales,” Vecchierelli added.

As a general rule, for events lasting up to four hours, provide at least one portable restroom per every 50 people. Go higher if alcohol will be served. For details on how many porta potties you need for different numbers of people and different event lengths, see the chart in How Many Porta Potties Do You Need?

If the weather forecast looks especially nice, consider bumping up your attendance forecast — and your portable restroom order. And don’t forget that you’ll need some handicap-accessible units.

Remember that looks count

Some local portable restroom providers will rent you older units or mismatched units, giving your event a shabby look or an eclectic look that you weren’t seeking. A larger provider with more inventory may serve you better. At United Rentals, the newest units are reserved for events so that they’re never a source of embarrassment.

Think about hand washing

Especially if the event includes food options, think about how guests will wash their hands. Consider renting porta potties with an in-unit hand washing sink. These units have a water tank and a foot pedal faucet. Other options are hand washing stations (plan to have one for every four toilets) and portable hand sanitizer stands.

Treat VIPs to an upgraded experience

If you’re treating important guests to a VIP event experience, or if the event is fancy or formal, make sure the restroom experience matches. Luxury portable restroom trailers offer all the comforts of a home restroom, including lights, running water, flushing toilets, mirrors, porcelain sinks and even laminate flooring, not to mention heat and air conditioning and space to move around. Luxury restroom trailers are available with anywhere from 2 to 14 stations and are the perfect choice for outdoor weddings.

For advice or help with renting portable restrooms for your event, visit United Rentals or call 833-548-4536.

Marianne Wait is an editor and writer who creates content for Fortune 500 brands.