There seems to be much hype and little technical detail about this 'problem'. Some more details here:
http://www.google.co.uk/search?q=cranfield+evil-twin
Whatever, I'd recommend the following as a minumum:
1) Start by making sure your wireless network employs WPA-PSK encryption. Make sure the pre-shared key is long and random.
2) Switch on MAC filtering to ensure that only your own devices can get into your access point.