MAKING IT MORE POTENT??!!

How do I make a not so potent strain into an even more potent?
My friend has 3 week old plants, but he said that the herb he got the seeds from wasn't as potent as other finer strains. I just wanted to know what sort of nutrients or ferts. he can use to obtain a higher THC level. They're 3 weeks like i said, and are being grown outside.

Also I heard another friend talk about an additive that helps the buds bud bigger. He said he read it in High Times, but i don't remember the name of it. I

Thanks!