There is some suggestion from people on the web that the source of the henna may influence efficacy.
from the author:
Here's the quick version:
Buy henna, preferably Jamali. Start with maybe a 50 gram box. In the US I recommend castleart.com. However, a site in Canada is giving a free sample of Jamali to anyone who sends an e-mail saying they have HFS. The address is hennasooq@gmail.com.
How to mix it
Bring a quarter of a cup of water to a boil. Take it off the heat and Mix in the henna powder until it is like cream of tomato soup. You’ll probably use 1/8 of a cup. Add a squirt of lemon juice.
I use a foam paint brush to apply it. I apply a thin layer to my barefeet. It takes maybe 15 minutes to dry. I then put socks on and leave it, but you can also rinse it off.
Henna stains, so I put a plastic bag and a towel down while it’s drying.
You can do this every few nights, or more often if you need it.