An abstract is a short but fully-contained summary of a study. Its purpose is to entice readers into wanting to read the full paper. This poses a challenge, given the ever increasing volume of documents and texts. As such, an effective abstract is key for the dissemination of your research.
An effective abstract should be “highly structured, concise, and coherent” .
Its structure should mirror that of the full paper, including the following sections:
Introduction: Provide a basic introduction (1–2 sentences) to the field that is comprehensible to scientists from any discipline, and a more detailed introduction (2–3 sentences) that is comprehensible to scientists in the relevant discipline and which introduces the gap in the literature .
Aims: State the hypothesis of the study (1 sentence).
Methods: Provide a brief description of the experimental design (1–2 sentences).
Results: Clearly state the most important finding, in both subjective terms (We found that treatment X resulted in a higher survival rate than treatment Y) and in the form of real data. Explain how this result compares to what was originally thought (i.e. was your hypothesis correct) (3–4 sentences) .
Conclusions: Explain the implications of your results and why they are important (1–2 sentences). 
Abstracts with this structure reflect good scientific practices. If you carried out the study with clear objectives, the abstract should be easy to write. The art of abstracting (Cremmins, 1996)  How to construct a Nature summary paragraph (http://www.nature.com/nature/authors/gta/index.html#a1.2)