Quit Smoking
Nicotine constricts the arteries by up to 30%, and that results in 50% less blood flow. 

Shed Excess Weight 
Being overweight also restricts the blood flow to the penile tissues. 

Lower Cholesterol level 
The arterial walls of the penis carry the blood flow to the cavernous spongy tissue. The arteries of the penis are very small and fatty deposits can reduce the blood flow to the tissues, thus preventing the full size from being obtained. 

No Cocaine 
Stop using it. In a short time, cocaine can make the penis arteries of a 20 year old, have the look and pathology of a 75 year old man. 

Follow instructions 
Stick to the directions given by the exercise program you are using, or take penis enhancement supplements as directed. 

Develop a routine
For your penis enlargement program. And stick to it. 

Focus
Do not join 2 or more exercise program at the same time, or take 2 or more penis enlargement supplements at the same time. (Note: it is ok to join 1 exercise program and take supplement at the same time as they work synergistically) 

Do not over-stress your penis
By overdoing penis exercises. If the instructions is 30 repetitions, stick to it. Doing additional 30 more does not mean you penis will grow 2 times faster, it could means bruised penis instead. 

Keep track of your results.