Update
As Aaron points out, there is the chance the copy-and-truncate can miss some entries. So the safer method is to move and FLUSH.
Original
This article has the basic principle to rotating the slow query log that I use. Basically you need to copy the slow log to a new file, then truncate the contents of the slow.log:
cp log/slow.log log/slow.log.`date +%M`; > log/slow.log
If you just move the slow log to a new file and creating a new 'slow.log', it won't work because the moved file still has the same inode, and mysql still has it open. I suppose moving the file and then issuing a FLUSH SLOW LOGS command would work, as that closes the file and reopens, but I find the copy-and-truncate to be just as effective and doesn't require logging into mysql.
His article mentions using logrotate in Linux, but I just made a cronjob to run once a day at midnight to do this for me.