Looks like the Next VIP slots are now being sent out - Booked mine for Saturday @ 18:00
Go to http://www.next.co.uk/vipsale
This takes you to a login page...
Login, then you should get a page that lets you chose the slot you want.
I guess it if your not a selected as a VIP, you won't get the page to let you book a slot.
Once you have booked a slot you can view the sale listing - http://www.next.co.uk/viplist/