Chisel blade

My tool of choice is an old Exacto chisel blade, which I resharpen on some 600 grit sandpaper as needed.  It is important to have it sharp, because that will make it cut instead of tear, meaning less effort and batter control.  As for the old part - for one thing I'm cheap, but as you re-sharpen it you tend to round the corners slightly, and that's a very good thing as it prevents them from digging into the body.

If the blade is too wide, it can be modified with a Dremmel or similar.  Just be careful, as little sharp bits of metal flying around are not good for your health.

Tools

Micro Mark has tools for this. Look under, Carvers, Scrappers, & Chisels. They have all kinds of tools for modelers.

Mission Models' has the Micro-Chisel which is very nice and has replaceable tips.

Rich

Convex Blades

I like using blades with a convex curved shape.  Scalpels are useful if you can get them, and X-Acto has similar shapes available.  The rounded blades are less likely to gouge the plastic than ones with sharper corners.

Many Thanks

Thanks everyone for the great answers.  This is my first "whack" at serious detailing, so all the help you give is most appreciated.

Recently did the same

A couple of weeks ago, I posted steps I followed as a few Accurail hoppers were upgraded. It's the second feature down on my blog here:

http://designbuildop.hansmanns.org/

I have some dental tools that have come in handy, and a sharp single-edge razor blade has become my favored cutting instrument. Between cutting and scraping with these tools, cast-on grabs can be removed easily. You may encounter some problems with your first efforts, as that is a part of the learning curve. Go slow, control the force you exert, and take a break after doing a few. It's not a race.

Best of luck!

Eric