Rogaine and Retin A have been proven to help eyebrow hair growth. I have not used Rogaine, but I use Retin A all the time, and have for years. Since it gets on my brows, I have noticed it does make them thicker. You can do a web search and read up on it.
You can use these togther or seperate. You can use the cheaper store brand of Rogaine and try that. Use the men's version, it's a bit stronger. Just be careful and keep it out of your eyes of course.
It won't work overnight, but it should help.