Writing an abstract
The abstract is probably the most important part of a paper. Many readers will not read anything else, so you need to grab their attention and get your main message across as clearly and succinctly as possible. It is not meant to be an introduction to the paper, but a summary of the paper. In a single paragraph, a reader can learn the purpose of the research, your general approach to the problem, your main results, and the most important conclusions. Write as if you have one minute to explain the paper to an interested colleague, assuming that she will not read the paper herself.
A good abstract will contain the following elements.
What did you do?
Why did you do it? What question were you trying to answer?
How did you do it? State your methods.
What did you learn? State your major results.
Why does it matter? Point out at least one significant implication.
An abstract should stand on its own, and not refer to any other part of the paper such as a figure or table. Abstracts generally do not have citations either.
Other things to keep in mind:
As a summary of work done, it is always written in the past tense.
Be explicit, precise and concise.
Stick to a single paragraph.
Restrict background information to a sentence or two at most.
Make sure that your abstract is consistent with what you reported in the paper. (This is particular important when a paper has been revised and results may have changed.)
As a general rule, write the abstract last. After all, you can’t summarize something that is not yet written.