Paris is one of the world's most visited cities—with crowds to prove it—so it pays to be prepared. Buy tickets online when you can: most cultural centers and museums offer advance-ticket sales, and the small service fee you'll pay is worth the time saved waiting in line. Investigate alternative entrances at popular sites (there are three at the Louvre, for example), and check when rates are reduced, often during once-a-week evening openings. Also note that national museums are free the first Sunday of each month. There are many within Paris, including the Louvre, Musée d'Orsay, and Centre Pompidou.
A Paris Museum Pass (www.parismuseumpass.com) can save you money if you're planning on serious sightseeing, but it might be even more valuable because it allows you to bypass the lines. It's sold at the destinations it covers as well as at airports, major métro stations, and the tourism office in the Carrousel du Louvre. The two-, four-, and six-day passes cost €42, €56, and €69, respectively.
Stick to the omnipresent ATMs for the best exchange rates; exchanging cash at your hotel or in a store is never going to be to your advantage.