An abstract provides an outline of the major ideas in a document and preserves a sense of its organization. Good abstracts also capture a sense of the language patterns of the original document. But most importantly, abstracts are brief -- usually no more than 100-250 words.
See page 717 of The Scott, Foresman handbook for Writers, 4th ed
"While conservation groups, ecologists, hikers, and equestrians want to keep mountain bikes out of parks and wilderness areas, mountain bikers consider these lands--especially single-track hiking trails--their natural environment. If mountain bikers wish to use trails in public lands they must behave responsibly and organize politically to defend their rights. The war between cyclists and environmental groups is due in part to the explosive growth of mountain biking and to the sometimes outrageous behavior of cyclists in sensitive areas. fortunately, cyclists have begun to practice more responsible trail riding. Their efforts have won them recognition and approval from important environmental groups and the United States Forest Service."
- Summarize the major ideas in the original document
- Write in a style that reflects the wording of the original text
- Don't quote from the original document
- Remain consistent with the original document's structure and organization--even if you can organize the material better
- Check conventions--the format of an abstract is dependent upon specific documentation styles (APA, MLA, CBE, etc.)