If your items are breaking when dropped, first you may not have baked the clay thoroughly
(...are you using an oven thermometer to measure the correct temp, and baking long enough for the thickness of clay you have, for example?).
It's also possible that any "joins" in your item (heads/arms/etc. or onlays, etc.) weren't made quite well enough so they're breaking off at those spots
Also, just not mixing one or more clays together well in some ways can keep them from bonding together well.
The most common reason for breakage though is using one of the clay brands/lines that's brittle and weak after baking wherever it's thin
(those would be Sculpey, SuperSculpey, Sculpey III... and FimoSoft to a lesser degree).
If that's the case, you may want to use a stronger brand/line of clay so that the baked clay will be stronger even when it's thin, and later stressed (those brands would be Premo, FimoClassic, Kato Polyclay, Cernit).
If you're making rounded and/or compact beads or other items which have no thin or extending areas, even the weak clays will be strong in those inherently-strong shapes and shouldn't break (unless under-cured).
You can read more on the differences between brands/lines of polymer clay, making strong joins, and baking clay "thoroughly" on these pages at my site if you want lots more info:http://www.glassattic.com/polymer/Characteristics.htm http://www.glassattic.com/polymer/glues-Diluent.htm
(... click on Some Bonding Techniques