Because of the Field Museum's size, crowds inside aren't as big of a problem as they are at other Chicagoland museums. The two main issues with crowds are getting in, and getting food.
Food is the easiest to solve: arrive early. Seating at the Field Bistro is quite limited, so certainly follow my general guideline of arriving by 11:30 to a museum restaurant, and maybe even aim for a little earlier. 11:30 should be a good time frame for the If you're bringing your own food, starting by 11:30 is also a good plan for the Explorer Cafe. If you want to eat a little later at the Explorer Cafe, at least keep an eye out for the 3-D movie times, since the theater is right next to the cafe—crowds will often go directly from the movie to lunch.
For getting in, the lines can get pretty long on busy days, especially summer weekends and days when school is out (MLK day, spring break, etc). One option is to buy tickets online ahead of time, though this includes a 10% fee. If you've bought a ticket online, you can skip the main line and go to the Will Call desk on the far right of the south entrance (the main entrance, other entrances don't have a Will Call). Another alternative is to buy a membership: it only takes one or two more visits that year to save money, and you can skip the line and go to the membership desk (even to buy the membership for the first time). Note that there is a separate membership desk at the south and east entrances, but not at the north entrance: you can still buy or use a membership at the north entrance, but it doesn't let you skip the line. Lastly, you can buy a City Pass if you're planning to visit multiple museums in a few days, these tend not to save you much money, but the Field Museum comes with VIP admission (this appears to just mean that you can pick up your tickets at Will Call). If you don't want to spend the extra money to do online tickets or memberships, and the south entrance line is long, consider walking around to the north entrance where the wait is likely a little shorter (the line is typically much shorter, but with fewer booths to get in it also moves more slowly). If you have a stroller, wheelchair, or otherwise need an accessible entrance, the east entrance often has a shorter line as well.
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