I know there are bagpipe forums, but nothing beats X Marks for speedy replies

I have just started out on the prcatice chanter, and with no previous experience of playing any instrument, I am quite happy with getting through Amazing Grace after a couple of weeks.
But, I can't get through more than two or three phrases before my lips pack it in. I have plenty of air, but how can I build up stamina for my lips?

Thanks