Body odor is not actually caused by the body itself. Rather, it is the result of bacteria which live on the skin and feed on skin oils and sweat. As these bacteria break down and digest the skin's secretions, they produce byproducts which give the bad odor.
Based on this, the two things that can cause an increase in body odor are an increase in bacteria on the skin and an increase in the production of sweat.
To combat the former, taking regular showers with a good anti microbial soap and changing undergarments if they are frequently saturated with sweat can help. To combat the former, using a strong antiperspirant deodorant will often do the trick.
You should also discuss this issue with your primary care doctor
. If your sweat production is becoming a major issue and is hard to control, there are actually a number of prescription strength products which can be used to dry out the skin and reduce the amount of sweat it produces.