Get them involved. Go beyond tea-making duties or trips to the dry cleaner, and treat them as a team member. It means a richer experience for them, and the extra hands can free up valuable time for your staff.
Explain things. What's bread-and-butter to an old pro like you may be real sweat for the inexperienced. Find five minutes to explain a task properly; that way, your workie won't end up creating extra work for you.
Praise them. When they do something for you, thank them and give feedback. It may seem hard to find the time, but at least you'll both know they're doing the right thing.
Pay them. Even a token gesture (£50 for a week?) will be an unexpected bonus for someone seeking only experience. But be sure to check protocol with your HR department - you don't want to get collared for exploitation.
Take an interest. Ask them what they want to achieve, and tell them anything you know that can help them.
Give them a leg-up. If they've got potential, let them browse vacancies on the company intranet. And if you have real faith in them, recommend them to contacts in the right places.