I know eyebrows frame the face and you should groom them to shape best for your face.
I also know how much work that can be hence why I favour a full fringe haha!
In all honesty I have only just recently started making an effort with eyebrows
and although I know most gravitate to filling their brows in with powder of some sort
I very much prefer using a pencil to some what tame mine.

Naturally I have an odd shade of hair it is very close to that of supermodel Giselle's
(sadly that is where the comparisons to such model start and end),
it is an in-between shade neither blonde nor brunette so finding a subtle shade to suit 
my natural hair and brow colouring can be somewhat troublesome!
Most brown shades are too deep and look ridiculous other too light and look ashen
the best shade I have found is 17 Eyebrow Shaper in Natural Brown.
it gives my brows a little definition without overwhelming my face.

The formula is not the best admittedly but you can make it work with a little effort.
The main problem with the pencil is that the formula is a little firm and can be painful to use
if not warmed up before use but let's be honest that is not a hard task is it?
Once warmed it glides on to the skin and gives a lovely natural effect,
it is also very easily blended and wears all day without fading nor smudging.
The great thing is these pencils sharpen like a dream and last forever,
I've only had to replace mine through sheer carelessness not through usage!

So how do you fill your brows?
Powder or pencil?

Availability - 17 Eyebrow Shaper in Natural Brown, £2.99/Boots