Take a 2 liter bottle, cut it open, and line the inside with some sandpaper from the hardware store. Then, put her in it and just slowly change the axis of the bottle by rolling it around and rotating it and turning it so they have to keep walking and gripping it. I think this'll wear their nails down in not too long a time. I also think this was originally mulder's(?) idea...before I heard this I wondered how to do it myself.
edit: Oh, I think you can also bring in a rough rock from the garden that they would enjoy climbing all over...I know people who keep rocks in their cages and it keeps the rats nails at an acceptable length.