I smear a bit of engine oil on the gasket from time to time.
The other thing to check are the cams inside thae upper surface of the tank. Heavy-handed pushing on the cap can crush the tank opening in a bit over time. This will be difficult to see but if you lay a straight edge over the tank gasket sealing area of the tank you'll probably see a miniscule amount daylight under the inner radius of the opening.
This is easily corrected by using a long screwdriver with the shank wrapped in tape and gently working any off-level portions of the cams until the cap fits snugly, but not overly tight.