Researchers from The College of New Jersey trained subjects take betaine (aka TMG/Trimethylglycine) in a sports drink and measured the number of reps the subjects could perform in the squat. After two weeks of supplementing with betaine, the subjects were able to complete eight more reps with the same weight, while those taking a placebo only increased their strength by three reps.
Supplementing with betaine helps the body in three ways. First, it promotes the natural production of creatine in the body. Second, it helps by lowering lactate levels that lead to higher acidity in muscle, which leads to fatigue, so keeping lactate levels low can prevent fatigue. Finally, it seems to boost growth hormone.
You can get it naturally be eating more wheat germ, quinoa and spinach. Sou should be eating these frequently. When you don't happen to eat any of those, take a natural beatine supplement.
There are two types beatine HCL, and beatine TMG. You want to get the TMG version. The HCL is a very acidic digestive aid.