A thesis is the main point that you will be arguing along with how you are going to argue your point. The rest of the paper will flesh out the argument. An abstract is a smaller form of this. It should be a paragraph long usually, but this can change only with good reason. In the first sentence write out the main argument and use the rest of the sentences to go over the main points of your arguments. If you are using a theoretical framework or something else that the reader should know, put it down.
Finally, consider your abstract finished when you cannot take anymore out, not when you cannot put anymore in. Simple is best for an abstract.
Steps to writing a thesis abstract:
1. Write the thesis first. Complete thesis should exist to write an abstract.
2. Have it read by at least three reviewers.
3. Get the reviewers to write the three most important aspects of the thesis, in no more than three lines.
4. Give each reviewer the aspects the other two have written and ask them to rank these aspects (just ranking, no remarks, no opinions).
5. Ask each reviewer to revisit their original aspects and rank them in order of:
a) "relevance" (to the thesis subject),
b) "significance" (in terms of the implications of the thesis), and
c) "validity" (in terms of what the thesis asserts or defends).
6. Collect all the work and evaluate them as follows:
Extract all aspects written by the reviewers. Now each aspect will have a ranking in each of the relevance, significance and validity categories. Evaluate each aspect according to the following scale:
Relevance gets 50% weight, significance gets 30% and validity gets 20%.
Top ranking for each aspect gets 100 points, second ranking gets 50 points and third ranking gets 0 points.
Since each reviewer ranked the aspects provided by the others, each aspect will get three different points according to the independent ranking by the three reviewers.
For each aspect, multiply the ranking points with the corresponding category weights.
Add all points and rank the aspects according to their total points.
7. Sketch the abstract with the following guideline in mind.
a. The top ranking aspect is what the abstract will be about. Half of the abstract text will be on this subject volume wise.
b. The second and the third ranking aspects will be the supporting factors of the thesis. A third of the abstract will be on these subjects volume wise.
c. Other aspects will be the supplementary points in the abstract. The rest of the abstract will mention about these subjects volume wise.
8. Write the abstract in a clear and concise way briefly elaborating on the relevance and the significance of the top ranking aspect. The text should then cover the supporting factors and briefly mention about the other supplementary points. Finally the text should conclude with a statement about the validity of what the thesis asserts or defends.
Also the abstract should lead the reader to a reference to get further information on the subject.
Finally remember that plain language and seamless flow are the two musts for a good abstract.