How to get students to highlight academic text more effectively?

Don't.

Dunlosky, John, et al. "What works, what doesn't." Scientific American Mind 24.4 (2013): 46-53. -- What Doesn't Work:

HIGHLIGHTING

Students commonly report underlining, highlighting or otherwise marking material. It is simple and quick — but it does little to improve performance. In controlled studies, highlighting has failed to help U.S. Air Force basic trainees, children and remedial students, as well as typical undergraduates. Underlining was ineffective regardless of text length and topic, whether it was aerodynamics, ancient Greek schools or Tanzania.

In fact, it may actually hurt performance on some higher-level tasks. One study of education majors found that underlining reduced their ability to draw inferences from a history textbook. It may be that underlining draws attention to individual items rather than to connections across items.

WHAT YOU SHOULD DO INSTEAD: Highlighting or underlining can be useful if it is the beginning of a journey — if the marked information is then turned into flash cards or self-tests. Given that students are very likely to continue to use this popular technique, future research should be aimed at teaching students how to highlight more effectively—which likely means doing it more judiciously (most undergraduates overmark texts) and putting that information to work with a more useful learning technique.

Emphasise that there is no right or wrong method

I don't think it is worth giving a prescriptive set of rules on what to do and what to avoid. Learning is a very personal process and what works for one person may be useless to another. I think the most useful thing that a teacher can do is to describe as many different techniques or variations as possible, and let the student decide what works for them. It may be that highlighting the whole text just helps them to read by providing something for the hands to do. I believe some dyslexic students find the bright colours more helpful than a black and white page.

Encourage your students to examine their own learning process.

Encourage them to think about what it is they are trying to achieve via highlighting - help reading in the short term? Long-term memory retention? Make reviewing the article at a later date easier? But hopefully, if the purpose is to pick out key points, then by making them stop and consider this consciously, they will be able to draw their own conclusion that highlighting an entire page is not helpful.