The following are the requirements for a U.S. passport holder to obtain a visa to El Salvador:

1. Your valid U.S. passport. You must have at least 6 months of validity left on the passport beyond your intended stay in El Salvador. You must have at least one blank visa page. Amendment pages do not count.

2. One Visa Application Form for El Salvador, completed in full. If the visa application is not completely filled out, signed and dated, we will not be able to submit your documents to the embassy, which will result in significant delays in the processing time.

3. Two 2" x 2" passport type photographs.

4. Flight itinerary or copies of your air tickets.

More information:
http://www.elsalvador.org/home.nsf/consularinfo