i have serious issues with my brows. lots of hairs, but they're ridiculously fine. so they appear incredibly sparse on my face, especially against my really dark brown/black hair (and i'm naturally tan!)... not good. they HAVE to be filled in, otherwise it looks like i have no brows!
POWDER vs. PENCIL
i prefer powder over pencil. i've used pencil in the past and found that the drawn-in areas didn't look as natural. even with short, feather-like strokes, sometimes there would be a weird sheen. however, i still use pencil for the tail end of my brow.
i like to fill in my brows with clinique's brow shaper in charcoaled with a MAC 208 brush - using short strokes and taking a step back every once in awhile to get the whole picture. it looks super natural on me because it's not a black, but more of a dark brown. black can come off harsh, so i find it best to fill them in with a dark brown. [rule of thumb: always go slightly lighter than your hair color]. after the powder, i brush on bare escentuals brow gel to set the color and keep them in place. the whole process takes about 5-10 min, but completely worth it to open up my eyes.
as for maintenance, i try to keep them tidy with plucking after i've been shaped at a salon. i will usually grow my brows out every three months, then go in to get them professionally reshaped.
HTH!