Lots of eyedrops - artificial tears without preservatives.
Frequent breaks with "distance focus" exercises - basically look at objects out of the window, etc. to unclench the tiny muscles that you use to focus for near reading
Drink lots of water - getting dehydrated makes your eyes feel "gritty" and hurt.
Use bright room lights - so your tablet doesn't hit your eyes like a spot light.
Use the largest practical font size - again to reduce the work your eye muscles have to do to focus.
Some people find reverse fonts (white text on black background) to be easier on the eyes.
[Yes -
- I do lots of reading .... why do you ask?]