How can I make my modeling clay a little bit harder?

keep it cool/ put a fan on it/ open the window/ put it in the fridge/ away from hot lights.

Ive been in ceramics class for a year and we make modeling clay. What we do when it he clay is too soft is that we put a plastic bag/ cover on it and leave it there for one day.. Then it gets hard the next day.. But i also think the first answer here is good! Hope this helps. :-)

leave it out so it could dry out a bit and harden

True "modeling clay" will never harden and also will never "dry" since there's no water in it. It will firm up some when it's cool though. Some brands of modeling clay (plasticine) will work better than others under warm lights or with warm hands and manipulation though --ones like Plastalina and ClayToon, for example, which are also precolored (possibly not sold at places like Walmart, but at craft, hobby and/or art supply stores). The brands that work least well are the cheapie ones sold by Rose Art, etc, for little kids. You read more about those brands and doing stop motion in my answer here: http://answers.yahoo.com/question/index?qid=20100303074837AAm5Mew And my answer here has more about the kinds of "clay" there are and what makes them different: http://answers.yahoo.com/question/index?qid=20100621051506AA53JtD If you want more info about "leaching" out some of the oil, you might want to check out this page at my site: http://glassattic.com/polymer/Conditioning.htm The category called *Leaching* there has to do with polymer clay (which is the other kind of oil-based clay, though it *can* be hardened) since polymer clayers sometimes leach their clays too.