Similarly, if you advertise a local business in Portland, Maine you would want to negative match "oregon", "or", and any other state that has a Portland.
Let me say first that I'm not picking with PMTC's good advice - I realize the Portland scenario was just an example.
However, I see a classic case worth noting in that example where you can shoot yourself in the foot, as I've done from time to time.
While you indeed want to apply -keywords freely, sometimes you may have to absorb a few bad clicks/impressions rather than exclude too many potential good ones.
"or" is a good example.
No, the Maine widget retailer may not want Oregon folk, but will also miss queries like:
"widgets in portland maine
or nearby"
"widgets for twenty dollars
or less", etc.
People really do search with queries like that and some of your broad or phrase matches would pick them up.
I've learned not to jump too quickly and exclude a word based on a few instances if the word has more than one context. Because I've been there, done that.
Atun