Here are some tips to help make the most of yours;
Arch the Right Way: look closely at your Avi's face. Imagine holding a pencil straight up and down right along the outer edge of your iris. The point where the pencil hits "her" brow is where your arch should be.
Stopping Point: The eyebrows need to end at the same place where the brow bone does. If they're shorter they'll look oddly rectangular. Too long? You run the risk of looking droopy eyed.
Lighten Up!: A lot of women use eyebrow tattoos, or eyebrow skins in entirely the wrong shade, either too dark or absolutely different then the hair "base" color. A good rule of thumb is that you want to go with a color that's a shade or two lighter than the color of your hair. (and remember, go with what is the color of the hair base, not what makeup you have on your avi.
Here's a link to some Skin sales that have the right approach to eyebrows + skin tones + hair base!
below, on the left side of the picture are two pictures of contradicting eyebrow tones and hair colors, on the right I switched the eyebrows, did a little work on them in GIMP....... and you be the judge!