You would have to use an aromatase(increases estrogen) inhibitor along with a SHBG(lowers free T) inhibitor.
Free Testosterone wants to become one of these 3 things when it is floating around in your body:
1.)
DHT
2.)
estradiol
3.)
bind to SHBG(Sex Hormone Binding Globulin)
Since Propecia inhibits the production of DHT, the Free T will mainly only convert to estradiol(type of estrogen) or bind to SHBG. To prevent conversion of Free T into estradiol you would need to use an aromatase inhibitor. An effective natural aromatase inhibitor I would recommend is Grape Seed Extract.
This should also help prevent some of the Propecia side effects such as gyno which is caused by increased estrogen levels. A potent brand of GSE I would recommend and use myself is Nature's Plus:
http://www.amazon.com/gp/product/B00014 ... B00014G47S
or Jarrow's formula is another potent brand I would highly recommend:
http://www.amazon.com/gp/product/B0013O ... B0013OXA2M
You also need to prevent Free T from binding to the SHBG. To accomplish this I would recommend increasing your protein intake and using Fish oil supplements.
This should help prevent sexual dysfunction by increasing testosterone levels.
This should result in a greater increase of Free T in your system which has many great health benefits.