I'm in the process of creating some icons for an application, and find myself coming up with the same question over and over again. Are there any guidelines for how thick the lines should be in an icon. I need to use lines with strokes that vary in thickness, but I don't know if there is any standard for it. Let's say the outline of my icon is 16pt, should the lines inside be ideally half of that? Is it better to use odd or even numbers? Any other tips or suggestions?
It will depend on the platform you're targeting and what the native icons look like. It also depends on the use. Are you creating icons for use inside an application, or the icon of the application itself?
Also, it's likely that you'll need to tweak each size of the icon. This is especially true for application icons and platforms like iOS, where many varying icon sizes are needed.
Here's some more info on techniques for scaling icons:
Even if the icons are for use within an application, scaling still may be a concern, because high DPI displays are about to become far more common.
And finally, I'd highly recommend against using Illustrator for icon design, if the final output is a bitmap (a PNG file etc). When it comes to generating bitmap artwork Illustrator is the wrong tool for the job. For the final output, anyway.