In Paris, it isn't just the leaves that mark a season's change. Starting in the spring, Paris is a vibrant, populous city. But in the summer, many Parisians leave for their own holiday. Every season is different, but avoid going to Paris in the summer and winter. Shop owners need a vacation, too. They usually take it in the summer and winter. You don't want your trip to northern France marked by closed shops and cafe owners on vacation.
Paris is a city so rich in the arts that you could probably throw a dart at a map and probably find a good museum, boutique and restaurant for the day. But to get the best out of your trip to Paris, you need to decide what you'd like to focus your sightseeing on. Paris is conveniently split into two halves: The Right Bank and the Left Bank.
The Right Bank is for the tourist who wants to shop 'til they drop, maxing out their credit card on the latest Parisian fashions. The Left Bank is for the tourist who wants to think and learn, complete with museums, galleries, concert halls and libraries. Knowing who you are and what you want to see will maximize your time in those specific venues.
How you're going to get around the city and where you're going to sleep in Paris is just as important as figuring out what to do during your waking hours.
If you're single, and you'd like to mark your trip with socializing and slumming it, buying a pass on the Metro system would probably be the best way to make great short-term friends. Also, the Left Bank is full of inexpensive hostels, or accommodations where guests can rent a bed in a dormitory and share a bathroom, lounge and sometimes a kitchen.
If you're a little older and/or married, bring a little extra cash for the taxi cabs zipping around town and the quieter hotels on the Right Bank. You won't meet as many people, but you'll be able to see more sights and sleep a little more soundly at night.