I have a tattoo on my ribs that is about 5-6 years old. Whenever I go swimming, etc. in the summer I have always put 50+ SPF on it. It still looks like it did after it completely healed the first year, not even exaggerating. So yeah, moral of the story: use sunscreen... forever.
When exposed to sun I use sunscreen too but only a physical zinc based sunscreen. Some types of sun screen penetrate the first layer of the skin and I am not so sure the chemicals would be beneficial to the ink.
Other than that I use coconut oil or linseed oil (for body and face too). Moisturizer increases water pressure in the outer skin layer, which can results in too much water and therefore dry out the skin and tattoo ;).