div0 was so kind to write a perl script that will slice your big recorded demos (that contain several map in a row) into several smaller demos, a new demo is created after each mapswitch. This way you can have demos of each map, and all the warming up sessions or waitings can be cut out easily, as you just need to publish the demos that matter (when zipping them for example)
You can download it here. The zip file contains the .pl version (for linux users) as well as a win32 binary for the windows users, who don't want to install perl. The usage of the windows binary is pretty simple: simply drag the demofile you want to split on the demosplit.exe file and select "open with.." (when using right mouse button for draggin, it may be called different, no idea how a english windows looks like), the demo will be split automatically.