A medical device includes a polymer stent crimped to a catheter having an expansion balloon. The stent is crimped to the balloon by a process that includes heating the stent to a temperature below the polymer's glass transition temperature to improve stent retention without adversely affecting the mechanical characteristics of the stent when later deployed to support a body lumen. A variable diameter sheath with a central portion that prevents expansion of the stent when the balloon is pressurized and larger diameter ends is disposed over the crimped stent-balloon assembly. The balloon is pressurized and the larger diameter ends of the sheath allow the balloon beyond the ends of the stent to expand. The balloon is then depressurized.