If you are asking this question, we have several suggestions for you. Johan Rooryck and Vincent van Heuven have drawn up concise and very helpful Guidelines. You will find them in pdf here (with the permission of the authors).
The LSA model
The LSA has a useful website on model abstracts.
SLE Award Winners
You can also look at the abstracts written by SLE Awards winners: SLE 2018 SLE 2019. They have been highly rated by our Programme Committee members. You can also consult the last year programme for inspiration.