Visitors need a passport valid for at least six months after their arrival date.
A round-trip or onward ticket is officially required of all non-EU citizens, including Americans and Canadians, regardless of whether you enter on the French or Dutch side.
St-Martin/Sint Maarten is a duty-free port, with no restrictions on items for personal (but not commercial) use.
Visas are not necessary for citizens of the USA, Canada, the EU, Australia and New Zealand. Some former Soviet states, Latin American countries and many African nationals will need visas, especially for the French side. Remember that different visa situations apply for the neighboring islands, so read up on the red tape before planning a day trip.