Is there work for them to do?
Don't blithely agree to have a friend's kid in for a fortnight without first checking that there's something useful for them to do - and a desk and computer for them to do it at.
Introduce them to the team
Make sure everyone knows the intern's name, how long they will be around for and what they will be working on.
Give them feedback
If they are doing a good job, say so. If they could do better, explain how things might be improved. Remember that they are new to all this, though.
Mix it up a bit
It's OK to ask the intern to do all that hideous admin that no-one else will touch, so long as that's not all you ask them to do. Try to line up a taste of the more exciting aspects of working life, too.
Look after them
Interns are by definition young and inexperienced - they need supervision. If you haven't got time to do that, delegate. But make sure someone keeps an eye on them.
Put them in your address book
Good interns can be some of the best-performing and lowest-risk new hires there are. So make the effort to stay in touch with them.
Do your homework
If the spotty youth in the corner who's just spent a fortnight on Facebook turns out to be the boss's favourite nephew, you'll wish you'd known sooner.