Here's a collection of useful articles that I've found over the years that give good advice on writing a good abstract, mistakes to avoid, etc:
- Adam Machanic - Capturing Attention: Writing Great Session Descriptions
- Gwen Shapira - Concrete Advice for Abstract Writers
- Kellyn Pot’Vin-Gorman - Abstracts, Reviews and Conferences, Oh My!
- Russ Unger - Conference Proposals that Don’t Suck
- Martin Widlake - Tips on Submitting an Abstract to Conference