A well-written abstract will maximise the chances of your document being read and understood. For a research paper a well-written abstract will maximise your chances of publication, and may be the only part of the paper that the reviewer will read. These resources contain some advice on writing abstracts.
For clinical trials:
This extension to the CONSORT Statement provides a minimum list of essential items that authors should consider when reporting the main results of a randomized trial in any journal or conference abstract. Clear, transparent, and sufficiently detailed abstracts of conferences and journal articles are important, because readers often base their assessment of a trial on such information.
This is a series of slides providing illustrative examples of randomized trials using the CONSORT for Abstracts Checklist.
For systematic reviews:
The PRISMA 2020 extension for Abstracts was published within the main PRISMA 2020 paper. The 12-item checklist gives authors a framework for condensing their systematic review into the essentials for a journal of conference abstract that will meet the needs of many readers.