To echo fm, one thing I learned early was that defining the parameters of a task carefully enough that you can hand it over to someone who is not a company insider and get back the results you want is actually a lot of hard work. It can be almost as much work as doing the taks yourself, and it is definitely not a trivial task in terms of time committment.
There is a temptation with interns and temps to think that they are just coming to do one particular thing, so you don't have to do the same kind of on-boarding, orientation, enculturation and team-building with them that you would have to do with a new permanent employee. My experience suggests that we need to do a bit more of this on-boarding than is generally thought, especially if we want the interns to "get" the business, and care about the business.
The unspoken assumption when we think about bringing someone new on is that we are bringing them in to do the task, kind of assuming that they will serve the culture of the company by instinct. I have brought interns in and see them totally not get this. From their perspective, the internship is a bit of monkey-work they had to do in order to get their degree, and what really interested them was going on at school. Since they never got fully oriented to and aligned with the project we had them on, and since we needed them to do something fairly complex, we essentially had to undo everything they had contributed and re-do it. Net loss for us across the board.
Like when you hire a permanent employee, hiring a temp works best if you take the care to select the right person - lots of prior work, good references, etc. In movies, they say 90% of directing is casting! If you get the right person (screening carefully to do so), then the job may be largely done.
But if you only have a good candidate, and not a perfect match, then up-front energy spent on-boarding this person is key. Once they start producing, careful task definition and early supervision and monitoring again is needed to get them rowing in the right direction. Understand all this, and ask yourself if there really is so much work that you need another pair of hands on the team. If you really do need someone to do this, remember that the up-front hit of getting them started right is better than mopping up a mess after them.
Management science studies suggest that even though this above thinking about the importance of up-front attention is true, most of us will still not do it, make off-the-cuff decisions to bring in temps and interns, and play the more expensive game of mopping up afterwards, because of the short-term illusion of convenience of having someone new to throw ownership of a task at. It's a predictable irrationality of human decision-making. That momentary illusion is so relaxing/reassuring that we will actually create more work for ourselves down the line than to do things right from the start. Fight that tendency!