Honestly I don’t do anything. Some of my tattoos have blurred too over time / ink spreading. Totally depends on everyone’s skin and tissue. Also depends on how deep the tattooist goes into the skin, how fine the lines are ect. 
The sun can affect tattoos for sure, I’ve only found the sun to bother my coloured ink tattoos personally as I get a skin irritation and hives on the ink. 
I occasionally put lotion on when I remember and sun screen in the summer, but mostly I do nothing. 🤣🥲🤷🏻‍♀️ 
I also never get my tattoos touched up like I should, I just end up getting new ones.😂🙃