In June we reported that YouTube had launched a dedicated service for mobile phones. However, it was and still remains very limited so you might find it a lot easier to simply download your favourite clips onto your PC and convert them to run on your phone.
YouTube Video to iPod, PSP, Mobile Phone allows you to do exactly this through an unglamourous but simple interface. As the name suggest, it's not dedicated to phones but it's one of the few really effective programs at converting to mobile phone video format. Within the interface, you have a field for an input file or a YouTube URL, the field for an output path, and lists with preconfigured profiles. Finally, in the list of devices select 'Mobile Phone'. Once you've got the output file, simply transfer it to your phone and away you go!