In a similar vein to previous guides, please shop around for lego sets. I've been looking for a new Star Destroyer for my son, most sets on e bay are going for over £70 + p&p, Amazon are currently selling for £69.99 with free p&p. I'm not saying they will always sell at that price but it's worth using a shopbot online to see what the going price is before bidding on an auction - thanks.
Update as at 19 Nov 07 - amazon selling at £63.99!!