Here are my tips for visiting Paris in the summer:

• Travel in August, preferably late August:  Most Parisians go on vacation at this time.  Almost everything remains open (small, family owned businesses may close) and most students are back home preparing for school.   The tourist attractions will still be crowded, however.
• Wait until about 60-70 days before departure to buy airfare.   Airfares are high to Paris this summer, the highest I have ever seen.  Unless booking with FF miles, you can wait until about 2 months before departure.   There are a couple of discount airlines run seasonal service to Paris like www.xl.com.
• Get a Hotel with Air-Conditioning:  Surprisingly,  a good number of Paris hotels do not have A/C.  Better check beforehand.
• Buy The Museum Pass:   This applies to anytime you are on a tour to Paris.  Skip the lines and return as many times as you please to an attraction.
• Visit Versailles on a Wednesday:  It's the most uncrowded day of the month.
• Don't Overdo It:  A hearty touring day is about 6 hours.   Schedule in some downtime at a cafe or restaurant.
• Stick a Relaxing Activity into the Middle of the Day:  A Seine River Cruise or a cruise down the Canal St. Martin is a great way to break up the day and give those feet a rest.
• Take the Bus, Not the Metro:  It's cooler and you'll enjoy the scenery much better.
• Visit the Main Attractions Late in the Afternoon:  Everyone else thinks like you and says "let's get there early and beat the crowds".   Actually most attractions (notably the Eiffel Tower) are least crowded in the late afternoon.

For more Paris travel tips and information on my small group tours to Paris see www.travelingprofessor.com