An Abstract is the most important part of a scientific paper. It not only summarizes the salient aspects of the paper but also lures a reader into reading it!
It is likely the majority of people who read your paper would only read the Abstract. If you want to attract more people to read your article, it is the Abstract that will do it.
The biggest mistake in writing an Abstract is to mention that such and such "will be discussed". The Abstract is not a place for waffling; rather it is a succinct summary of the exact details of your findings. The most important data and findings are contained in it, NOT left out.
Start out the Abstract by telling exactly what you did and how you did it. Focus on the rationale and ideas of the study and why it's important in the first two sentences. For example, "This study determined impacts on fish community structure by manipulating prey numbers in a tide pool in Oregon, then used Stella simulation software to model community dynamics".
In the next few lines, focus on the materials and methods, and the data generated from the study. Tell the reader how the data were collected, compiled, and state statistical significance(s). Mention any new tools developed.
Avoid using bibliographic references in the Abstract unless they are absolutely essential to understanding the scholarship or results of the study. However, if your article follows directly from a published work and is a major advance on that specific piece of work, do cite the paper in the Abstract.
The end of the Abstract is just as important as the beginning. This is where you want to hook the reader into examining into your paper! In addition, the concluding lines of the Abstract should lead into the first paragraph of the introduction without repeating what has been said. State the implications of your studies to the field of scholarship in which you are working.
Most of us wait until we finish writing the paper to write an Abstract. However, I suggest you make notes as you write the paper about the important parts of each section. When I'm done writing the paper, I read the paper over a few times, making a list of keywords as I read, then organize these (and my thoughts), and draft the Abstract.
With practice, writing good Abstracts will evolve from being a burden to a joy. You will see succinct summaries of your accomplishments appear. These will help you envision your next area of inquiry. In the process you will attract many people to your work; develop lifelong relationships; and contribute to the wonder of science.