I just did this a week ago so it's fresh in my mind.
Early on I realized that pulling with a single needle nose pliers helped to make the job harder. When you pull on just one vane, you can't help but put some directional force against the opposite side of the pump adding more friction and making it more difficult. That's when I found a cheapo needle nose vice grip tool like this:
http://www.amazon.com/Irwin-Tools-14.../dp/B00004YO5N
Now I lock one vane with that and use a regular needle nose pliers on a vane on the opposite side. That allows the impeller to be pulled more perpendicular to the housing. Adding some non-petroleum lubricant can help (I like Crisco) but I've always been able to just pull it straight out with this two-grip technique.
I'm also always finding uses for that needle nose vice grips.