I never thought twice about my brows until about two years ago. They were always full and thick. In fact, I was more worried about keeping them plucked and waxed then filling them in. However, age does a funny thing with eyebrows and suddenly, they're thinner than ever.
I've tried a few different options. Shadow with a small, angled brush is nice for a natural filler, but it's tough to get precise. Pencils give you a cleaner line, but can look extremely "painted on" and if the pencil is too thick, game over.
I recently discovered Jane Iredale's Retractable Brow Pencil and it really fits my needs. It has an insanely thin point, which allows you to get a precise, but also natural look. It also lasts the entire day without smearing or fading one bit. I'm using the "Brunette" shade, which is light enough to work with my dark blonde/light brown hair. I think the "Blonde" color would work as well.
Because the pencil is retractable, there's no need to sharpen it, which makes life a lot easier. Plus, it has a built in brush... they call it a "shaping spoolie"... that is really great for separating the brow hairs and getting everything in place. It works SO much better than the combo eyebrow brush/eyelash comb I normally use.
The website recommends light, short strokes to get a natural look, which really does work best. Avoid any long, heavy lines. After I've applied the pencil and shaped the brows with the "spoolie," I also like to go over the top with a light powder, to soften and set the look.
Jane Iredale's Retractable Brow Pencil is available on her website and at other retailers in blonde, brunette and dark brunette. It sells for $20.