If it's material you want to/are required to learn from try taking notes as you read the material, to force yourself to recall it in your own terms/language.
If it's just recreational/online reading try increasing the font size/spacing or decreasing the browser width, or using a browser extension like readability. Don't scroll with the scroll bar or the mouse wheel - use pg up/pg down to make it easier to keep your position.
I don't know if I deliberately developed a habit of highlighting the current paragraph when reading long articles, but it has become extremely useful.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.