If you are having trouble getting boys to do dance, play this on your Interactive Whiteboard whilst they are getting changed.

Don't forget to prime them first! Ask whether sportsmen dance; are rugby players considered 'soft' etc.

http://www.youtube.com/watch?v=8eGCsEQ15L4

or use this link...
http://video.google.com/videoplay?docid=-4415484536180232877&q=haka+google+video

Nick has also suggested using the following video, which shows the NZ Haka from 1973 which was pathetic in comparison. This should be used to show boys (in particular) why they have to commit to the dance as a token effort looks ridiculous.

http://uk.youtube.com/watch?v=KBCpPGNDr1U