I'd put a little baby oil on the bars and use a rubber mallet to encourage them into the stem. Maybe a little fine sandpaper on the clamp area of the bars would be nice. It takes very little extra material to make something not want to just fit perfectly; if you got a slightly too small stem and a slightly too large set of bars, you'll need to do some coaxing. I'm always pleasantly surprised when I get a stem and handlebars to fit perfectly; usually it's a lot of pushing, rotating, cussing, begging, etc.