Soleplate buildup can be caused by a variety of things: starch, fabric softener, hard water mineral deposits, even dust.  If your soleplate gets dirty, you can clean it quickly and easily with a wax-based iron cleaner, such as the cleaner in the Rowenta stainless steel soleplate cleaning kit.  Put about an inch of the cleaner on a towel and move the hot iron across it.  The buildup is transferred to the towel.  If the soleplate is very dirty, you may need to repeat this step.  when finished, buff the iron with an old cloth or towel to ensure that the iron is completely clean.