Castor oil will do the trick. Put some on your scalp every night or every other night and your hair will become thicker. There were some good threads regarding this. Castor oil helped my hairline a great deal. I also deep condition with MT under a plastic cap for an hour or all day once a week which has helped also.
There are protein treatments, adding castor oil to your dc and sitting under the dryer and also, there are volumizing conditioners(i.e., protein in it). That help with this.
Regular castor oil that you would find in the laxative aisle at CVS, Walmart or Walgreens. Some like the Jamaican Castor Oil, but to me, it does the same thing as regular for me.
Alot of ppl use Jamaican black castor oil, which I believe is available in some health food stores and online. But you can also use any old castor oil that you find in drugstores, grocery stores and it should work as well.