The first step to keeping that perfect white clay white is keeping your workspace clean and dust free. I often keep a bottle of rubbing alcohol and paper towel close at hand to give my work surface a good wipedown when I am using light colours.
Possibly even more important than a clean workspace? Clean hands. You'll be surprised at the amount of crud on your hands! Just plain old soap and water does the trick, but don't even bother using a towel to dry them off. Either let them air dry by waving them around like a maniac or by using paper towel - hand towels tend to be full of fuzzies and lint, which you'll see later when you start working with your clay. Trust me. A nice exfoliating soap or scrub brush can also work wonders.
Before you begin working with the white clay, take off a scrap piece and roll it between your hands. I do this for a few minutes, actually. You'll be surprised how imperfect that little scrap piece of clay will become. Better on the scrap than on your finished piece! And don't forget to roll it over your work surface, as well!
Are you rolling your clay? Rolling between two pieces of clean white paper or waxed paper works well. If there is any crud on your pasta machine or rolling pin, the white clay will pick it up - putting something between the rollers and the clay is a good way to avoid this.
Some people swear by latex (or similar) disposable gloves and finger cots when working with light colours. Personally, I find it difficult to sculpt with gloves but this doesn't mean that you might not find it useful.
Whether you like it or not, though, you're going to get *something* on that white clay, no matter how many precautions you take, but this can be fixed. Keeping a few cotton swabs and some rubbing alcohol nearby often does the trick. A little dab of rubbing alcohol on a cotton swab can be used to gently wipe the dust away. Just don't use too much alcohol, as too much can saturate the clay and make it hard to work with.
When all else fails - you've baked the clay but still discovered dust, sanding and buffing might be your only option. Check out my blog entry on Sanding, Buffing, and Polishing Polymer Clay. Others completely avoid the issues of keeping their clay perfectly clean by painting the clay, afterwards - just make sure to clean the areas you want to paint with rubbing alcohol first, use a diluted paint to avoid too many brush marks, and work in thin layers. Thin layers works a lot more better than one thick layer. Many folks like to use the dipping method as opposed to painting large areas with a brush.
I hope this helps. Have anymore questions about polymer clay and sculpting? Has this article helped? Comment! I'd love to hear from you.
Happy sculpting, everyone!