Non- compete clauses are generally evaluated on:
1) duration- most I know of cap at 5 years.
2) compensation- the more you get paid, the more restrictive the covenant can be.
3) scope- how big a geographic area you are prohibited from competing in. (For IP cases, this is virtually ignored, since it is essentially meaningless.)
4) breadth- in what fields can you be barred from competing. To use Mistwell's example, would an employee be barred from just formal wear, or in the garment making business as a whole, or in any job connected to garments (tailoring, retail, fabrics, etc.)
Basically,its all a big balance test- the courts want to make sure the former employee can still make a living, but without impacting the former employer's business too negatively.