Read the first paragraph, the headers and subheads, and bold words before you start speed-reading. After you read these, you'll be familiar with the text and can quickly skip over unnecessary information and more quickly find the information you need to know. This will improve your speed-reading. Skimming also lets your brain take in the text, so you'll remember it later when you're speed-reading.
Read the first and last sentences of each paragraph. Your skim reading will be faster if you are good at peripheral vision and can take everything in as a whole. Use your peripheral vision to look for sidebars or illustrations that can aid in your understanding of the text so you can decide what is important to you. Also skim the index of the text. Skim your eyes down the page and look for numbers if you're searching for dates or statistics.
Train yourself not to reread. This can be difficult, because you might catch on words and want to understand them better. Read the whole text through without stopping. Going back over the text is not the point of skimming. You're just trying to get the central ideas so your speed-reading will go smoother. You'll be re-reading the text anyway when you speed-read.
Read the last paragraph. This summarizes the key points of the article. Don't forget that skim reading shouldn't replace speed-reading; with skim reading alone you might miss relevant information in the text. If you don't have time to do both, you should speed-read rather than merely skimming the text.