An effective abstract is your first opportunity to hone your message.
Write a great abstract, pepper it with great visuals ... and, voilà, you have a great poster!
An abstract is a succinct description of your work. It should ...
Explain why your work is important - set the context as appropriate for your audience and pre-empt the "So what?" question.
Describe the objective(s) of your work. What are you adding to current knowledge?
Briefly explain the methods. Unless the research is about methods, this should not be a major focus of your abstract (or your poster).
Succinctly state results, conclusions, and recommendations. This is what most people want to know - your main message should be here. Do not say "We present the results of our study and recommendations for action" - tell them what you found and recommend!
We do not recommend including an abstract on your poster.
It is redundant, because a poster is already a succinct description of your work. Writing a good abstract, however, is an important part of having your work accepted for presentation at a conference.
An abstract can also serve as an outline for your poster, which can be thought of as an illustrated abstract. Write a great abstract, pepper it with great visuals ... and, voilà, you have a great poster!
Resources
Here are some resources from the American Society of Agronomy (with permission) to help you write more effective abstracts (and papers, too). This stuff may be old (2002), but it still holds.
Checklist & Sample Abstract: a one-pager that focuses on the qualities of a good abstract. You'll need to adapt this to your field, but the overall philosophy holds.
Be a Better Author: a two-page piece that deals with abstracts and writing in general, from the Jan/Fen 2002 issue of Agronomy Journal.