Your highlighting can make the difference between a 30-minute review and a 3-hour review.
When you've got a lot of material to cover, you don't want to be re-learning anything, and you definitely don't want to be scouring through your textbooks to find something crucial that you're missing.
Here are some tips for making sure that you're always on the right track when highlighting is concerned...
There are always certain important things you should highlight.
Depending on what you're studying, these may vary a little, but overall this list should come in handy.
You should highlight...

1. Dates, places and times.
   If you don't have these highlighted, they can be nearly impossible to find.
   
2. Important abbreviations.
   You will automatically recall the full name of the thing if you have the abbreviation highlighted.
   
3. Examples that represent the main context of the chapter.
   These are good for getting real comprehensive understanding of the material.
   
4. Series' of ideas.
   When a bunch of related ideas or concepts come in sequence, you should highlight them all (with the same colour highlighter, as I'll point out in a second).
   
5. Keywords, or words that are new to you.
   You will never forget them this way.
   
6. Important points.
   Make sure that you are highlighting as little as possible, without missing any of the important points.
   Don't highlight unnecessary words.
   If it seems like "everything is important," try highlighting the center of the sentence or the part which holds the vital information

It is a good idea to keep two or three different coloured highlighters handy.
This can be good for keeping colour codes for examples, ideas, dates, etc.
If you organize your highlighting properly, a quick glance at a page will be enough to find everything you need to know.