Aren't thin eyebrows in style? There's not much to do other than use brown eyeliner to darken the area.

tattoo them on. There some people that are very talented artists. I have seen the work and it looks awesome. At least your wouldnt have to draw them on all the time. For tattooin eyebrows they use ink that is your hair color, and they just fill in the blanks. The strokes look just like hair. Look online and do some research.

Get an eyebrow pencil and lightly draw them on. Then use a translucent powder to make them not be so harsh looking.
Just a hint...make sure you don't get your eyebrow pencil mixed up with your eyeliner...one of my friends grandmother used to pencil on her eyebrows...she used black which was funny in itself, but one day she mistakenly used my friend's blue/green. It will go down as one of my funniest moments.

Yeah tattoo them on it looks great! Or pencil them in... small upwards strokes to simulate the natural hair... There is a new product in Australia (and probably in USA) which is a gel and it has actual fibred in it and you put it on sparse or overplucked eyebrows and it fills them in ... I havnt tried it yet

As weird as this sounds..rogaine. I saw an episode of the Tyra show where she said a way you can fix too skinny of eyebrows was just applying a very SMALL amount of rogaine where you would like hair to grow.

Or like everyone has suggested makeup works wonders, I myself use a brow colour eyeshadow to better define my eyebrows.

good luck.

eyebrow pencil will do..