Good question. If you mean specifically getting around the "some objects on this page are insecure" popups, a trick I've seen work is to cut off the http protocol portion of remote URLs, forcing them to look like "//www.example.com/to/file.jpg".
That's just a link that's relative to the protocol, the same way /blah is relative to the protocol + host. If you load an https page with inlined image links like that, it'll attempt to hit those images over https as well.
This is fine if you control the source and can handle both http/https serving. It doesn't help for content inlined by users, though, it'll just result in broken images.
