The best thing to do is put these auto renew dates in your diary and 2/3 weeks before the auto renewal kicks in ring them up to cancel.
Miraculously they always have a "special offer or managers special on today" and ask you to hold on for a moment and they come back to you trumping any low price offer you've had elswhere.
Just did it yesterday on my home insurance and got 35% knocked off the auto' renewal price. They just rely on the unorganised to make their extra profits. Don't let emm' boys and girls!