Filling in a brow doesn't always have to look harsh. I like to use powders to fill in brows as I find it gives a softer more natural look than a pencil does. One brow powder I really like is the "Brow Blend" by Caryl Baker Visage. It comes 3 shades in 1 and can work for anyone from fair blonde's to dark brunettes which is why its not only great for personal use but for the pros too.
The brow blend which retails at all Caryl Baker Visage stores and online for $19.50 cdn is a little on the high side but if you consider that this product will last you over a year its not bad. Ive been using mine on clients as well as myself for about 2 years now and you can see below that its barely made a dent.
For my brows which are dark but sparse I like to use the medium shade for a really natural look. It doesn't look filled in or like I have product there if you didn't know it just looks more polished and it really completes any look.
(Left:Before, Right: After)
Something as small and easy as filling in your brows can make a huge difference in your overall look so take the time to explore natural ways of filling in those pesky spaces.
Other recommendations for great powders to use on brows are Omega shadow by Mac for blonde - light brown hair and Embark can work for darker hair. Remember to use a stiff angle brush like the one shown above and to apply shadow lightly and build as too much too soon will be hard to take off. If you do add too much try using the tip of a Q-Tip to remove some of the powder.
There are many options for filling in brows find the one that works best for you!