If you have a US or Canadian passport be prepared to pay 10 or 20$ for the visa at the border.
Also, it's a very slow process. Kids will hassle you to do the tramatis, and if you try to ask the custom's officers where to go and what to do, they'll sit back in their chairs and point to the kids.
First time through there it took me about 4 hours, and a couple of canadians even longer. There are rumours around that Americans get refused entry, but I never actually met one.
Unless you really want to go to El Salvador, you could follow the busier route south through Honduras, or Copan (the most beautiful and least hassle border crossing).
Everyone's packing a gun in El Salvador, but I didn't feel unsafe or come across any tourists who had problems - nor did I see any tourists.
Enjoy the puupusas!
[This message has been edited by t0by (edited 24 October 2002).]