Here's a relevant blog post from Gary @ VFTW
http://boardingarea.com/viewfromthew...ey-on-airfare/
Not commenting on the "ethics" of doing hidden city ticketing (I've never done it) but it boils down to:
1. Not against the law but against airline policy so they hate it and it may get you banned
2. The skipped leg should be the last leg(s)
3. Must be one way or the last leg of the RT
4. Don't put your FF#
5. Don't check baggage (or if you're brave, short check it)
6. If there are irregular operations (IRROPS), you're SOL and you will need to fly to the true destination.