As everyone knows San Salvador is not the safest place on earth, however you can manage to live safely by taking the following recommendations:
Avoid taking the public transportation as well as taking walks around the city.
When trying to find a place to live, look for an apartment or some place with its own security gate and security guard. Personally I think t the best places to live are “Escalon” in San Salvador and “Santa Elena” in Antiguo Cuscatlan.
And the most important thing, get together with locals! They know well the city and will tell you what to do in order to be safe according to your life style. The majority of Salvadorans are very friendly and will try to help you in anything you may need.
Hope this helps with your question!