The switchtheme module provides a simple way for users to switch between themes on your site. A block is created upon enabling this module that displays a select list for all enabled themes. Permissions for this block can be configured by roles, and it is a great way to allow guests to the site to demo different themes.
After enabling the module, simply set permissions for "administer switch" and "switch theme" and then just place the "Switch theme form" block in whatever region you would like. Under "Site Configuration -> Switchtheme Configuration" you can also give each theme a more user-friendly name that will appear in the select list.
Just make sure you set the "Switch theme form" block in the correct region in each of the enabled themes so that you don't switch to a theme and have the block disappear!