To enter El Salvador as a United States citizen you will need a current U.S. passport, and either a Salvadoran visa or a one-entry tourist card.
It is not necessary to purchase your tourist card before coming to El Salvador; you can buy a tourist card from the immigration officials at the airport. The cost for a tourist 
card is US $10 and the card is good for 90 days.
If you plan on staying in El Salvador for more than 30 days you can apply in advance for a multiple-entry visa, from the Embassy of El Salvador in Washington, D.C. or from a Salvadoran consulate in the United States. You may be asked to present evidence of U.S. employment, or show proof of onward travel or sufficient funds when you ask for a visa.
The official web-site of the Embassy says that United States citizens are granted the visa free, but the visa must be applied for in advance at a Consulate of El Salvador in the United States. If a visa is not purchased in advance, you must pay for the US $10 tourist card upon arriving in the airport.
When you depart from the El Salvador International Airport be prepared to pay an exit tax of US $27. If you depart by land or by sea from El Salvador the fee varies.
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