Permanent real gains in girth (I mean noticeable, like a full inch increase in girth), pretty much requires surgery (penile implants).
You can get temporary increases in girth (about 30 minutes worth) from penis pumps.
**** rings can also provide you with temporary girth increases by artificially increasing the blood pressure in your penis, which will increase girth. But as soon as you remove the ring, the increase will be gone.
Jelquing can provide temporary increases in girth, but usually no more than a 1/4" increase. Permanent increases from jelquing can also be had of about the same size, but requires a lot of time and effort, and again, can be dangerous if overdone.