Was in a similar situation recently. I work for a very large company and the person above me was leaving for a different position. They expected me to take on the load (which I was already taking on in expectation of getting the position when he left), while keeping my current position. All the while helping to train his replacement.
I walked into my manager's office and made my demands. My direct manager is a really good guy who completely understood everything I was saying. But at the time, I still didn't get the raise. My company basically gives you raises when you've reached new experience levels and that's it. I'm now casually looking around to see what other oppurtunities are out there. I think they've picked up on the fact that I'm not going to just take no for an answer, because now (3 months later) I'm getting an additional 2% on my yearly raise. Asking doesn't always lead to immediate results, but it's at least helped out a small amount over time. Good luck