CareerBuilder has some tips.
Even if they rave about you, six months may be too soon to ask for a raise. Wait a year. (Unless a lot of duties have been added in those six months.)
What about your year-end bonus? Too cheesy? Well, bonuses are not "givens." You can make a tactful inquiry. Maybe it was an oversight.
What if a new hire is making more and you find out? First, this person may have skills you don't know about. This gets very tricky--the whole equal pay thing. It's better to ask for a raise based on your performance--not someone else's check.
If you are offered too low an incoming amount--should you negotiate? Check out the Robert Half Salary Guide--sure, negotiate--it is often expected.
So your research, be gracious, pick your moment.