I cannot recommend the fabulous free plug-in AsideShop enough.

It’s simple to install, easy to use and allows you to set custom templates for as many post types as you have categories. Want video posts to look differently than image posts? With this plug-in, it’s very simple even if your theme isn’t custom post type enabled.

It’s one of the easiest ways possible to make your WordPress blog behave and look more like a tumblr blog.