Bongos are extremely durable and retain their playability very well. They are made with small, thick skins that do not wear out easily, and their shells are also thick. As long as their hardware is in good shape, you can essentially treat your used bongos the way you would a set of new ones.
Oil the skins of your bongos. Put about 1 tbsp. worth of almond oil in your hands, rub them together, and rub the oil into the skins of your bongos. Many bongo players forget to keep their drum skins moist, which is an effective way to keep them in good condition. Repeat this step whenever your bongo skin seems dry.
Loosen one drum head. Unscrew all four lugs 1/4 of a turn. Repeat until the drum head is loose. Then loosen the other drum head.
Look at the rim very closely. It should not be bent or warped in any way. With the drum head loose, it should sit evenly on the rim of the drum. If it is seriously warped, you may want to get it replaced.
Use your fingers to tighten the lugs on one drum head until they are tight enough to be difficult to turn. Go one at a time, tightening just a little bit each time. Even out the tension so they all have the same feel.
Tighten the head with a wrench by tightening all four lugs 1/4 turn at a time. After each 1/4 turn, play the drum to see what sort of tone it makes. You will know that you have reached the perfect tightness when you get a loud, clear tone.
Repeat with the second drum. The smaller drum (the macho) should be about three semitones higher than the lower one (the hembra). For example, if the macho is a G, the hembra should be an E.
Loosen the drum head when you are done playing. This will help help ensure that it lasts a long time.
Bring your drums into a music store when you first tune them. Ask them how high to tune your bongos. That way, you won't accidentally tune them too high and break the skin.