My wife and I are considering purchasing a home which had a termite problem in 1994. Other than a termite inspection, what precautions should we take. The owners said that it was treated. Is this usually successful? The house is in Michigan if that makes a difference.

An inspection is valueable, though it can't guarantee there are no termintes anywhere in your house.
Where I live, when you have your house treated for a termite problem, you get a guarantee. Then they usually offer you a package that includes an annual inspection (and if-necessary treatment) for a small annual fee.
The previous owner of the house should have some kind of document from the company that did the work, guaranteeing that the house was termite-free when they finished. Hopefully, they also have been getting an annual inspection since then. If they don't have any documentation, then maybe they they can at least give you the name of the company that did the work, and they might have records.
If they do have a contract with someone, chances are you can pick it up and continue it without incurring the large cost of an initial treatment.
If they don't have a annual contract, and the house gets a clean bill of health from the inspector, you ought to consider an annual contract with a reputable exterminator. The initial cost is quite a bit, but maybe worth the peace of mind, especially if you will want to sell the house sometime in the future.