You can get products from your LBS to apply inside the frame. It's a good idea to protect your investment, especially if you live near the ocean or in a humid climate. In NM it's like 0% relative humidity year round so I don't bother for my steel MTB, but probably should.
There are littles drill holes usually in the seat and chain stays. Fill'em with spray oil and swoosh it around a bit after wet rides. Other than that, wash it and wax it. Mines nearly 25 years old and still going strong.