What helps cook food evenly in microwave ovens?

Some microwaves will have what is called a carousel, a base that you place the item to be heated on, and it will rotate, ensuring that your item receives an even dose of microwaves on all sides. Your other option would be to purchase a commercial grade microwave that is more high-powered than those that are typically used in the home.

Microwave with carousel:

http://www.ikea.com/us/en/catalog/products/50142337

Commercial microwave:

http://www.missionrs.com/NE-1054.html

Originaaly, microwave ovens used a "mode mixer", located at the top of the microwave oven: it resembled a slowly rotating fan but with all the blades pitched at slightly different angles. The microwave oven industry switched to carousels (turntables) at least 10 years ago and they remain the uniformity-mechanism of choice. They are made of glass (except in combination ovens that have a grill) and also serve another very important function as a "matching device" where they actually absorb some of the microwaves when a consumer heats a very small amount of food or something that doesn't absorb microwaves very well, thereby preventing excess microwave energy from being reflected to and damaging the magnetron tube.