I tried scoring and it didn't help me. I finally bought the Taurus 3 ring saw just so that I could get the cuts I want without losing some of the design, especially when there is a design in the center of the plate. You might try the blue painter's tape; I think I read somewhere that it helps to direct the break.

Scoring didn't help me either. I don't know if the painters tape will help, never tried

This is the thing I have, but I rarely use it anymore in favor of my leponnitts. I've resolved myself to using 'not so straight' pieces, but have also got a ruler and a pencil-grip glass cutter to use for straights:

I use that tool on plates also, with good success, its called a tile pliers I think.
I can cut fairly straight strips from the plate most of the time, but depending on the footer, and how thick the china, its not always perfect.

I'll give the tape a try with my nippers and report back. I tried it with my wet tile saw, but the tape slipped right off as soon as it got wet. Thanks for your reports, everyone.

mmqchdygg, can you explain the ruler and pencil grip glass cutter? Do you use it on plates?

Wackyweeder, do you mean you use the tool she shows above or the glass cutter?

It depends on the plates. I've had some that I scored and got a perfect break along the score, and others that wouldn't break where I wanted them to no matter WHAT I did. I don't think there's any predicting it!