Skim reading is quickly going through whatever materials and trying to find keywords or topics. Before starting to skim through a document, think of the keywords and terms you are looking for, that way you have direction when you go into reading the document!
Also, remember to read the introduction so you get an idea of the writing style and things to come, this will only help prepare you for when information you need comes up and the structure of the work that you are going to read.
Another top tip for skim reading - always read the first line of every paragraph properly. The first line of, more often than not, explains what is going to follow in the paragraph. By reading that properly you can know whether or not it is worth skimming through the rest of paragraph.
Last but not least, when skimming, move your eyes vertically as well as horizontally. So basically, move your eyes down the pages as much as you would across the page. That way, you will not be able to miss a thing!