Nose in a Book has a good post about a recent trip to a science museum.
Back when we were first homeschooling we took a trip to the opening of a big exhibit at the local zoo. The flyer said that the opening day would be reserved for members only, so I figured we would have exclusive time with the animals, especially the elephants. However, when we arrived I knew it was going to be a zoo, literally. There were dozens of city schoolbuses in the parking lot, all filled with chattering, screaming, and out-of-control children. The morning was a disaster with my biggest concern not losing one of the kids in the chaos. After an hour I gathered my crew to hit the road and the local diner for lunch. We came back a few days later when there was peace and quiet.
My recommendations for field trips include, getting to the museum as early as possible, sit down and wait for the crowd to pass by (they only stay at each place for 5 minutes), and figuring out which direction the biggest groups are going and go the opposite direction. Schelduling trip at the very beginning and end of the school year usually results in only encountering other families. If crowds in general are not your thing then outdoor exhibits like Jamestown and Williamsburg are best enjoyed in the late fall and winter, not in the steamy summer months.
Also, if a museum is a big hit with your kids, like the Virginia Living Museum was with ours, buy a family membership. It usually is only slightly more money than one admission, and you can get newsletters announcing special perks and classes, as well as the freedom to visit anytime for an entire year. Some of our member specials included seeing special planetarium shows, gazing at the biggest snapping turtle in captivity, touching a blue tongued skink, crawling inside a LEGO sub, and patting pythons. Some were not my cup of tea, but the kids loved it and learned so much more than they would have if they had come in a big yellow bus.