Sign up for the baby clubs/Bounty etc online - you get coupons and freebies. Boots Parenting Club - lots of vouchers and extra points, and free changing bag with travel mat.
Agree with getting second hand stuff - where it's what you want/need and in good condition. A workmate has given me a load of stuff, best part of £1000 worth, all in great condition - she's practically kitted me out.
Asda and Tesco seem to be cheapest for basics - muslins, sleepsuits, babygros, etc. Asda give you a £5 coupon for any baby stuff if you pick up your Bounty bag there. Tu at Sainsbury does the best non-branded grobags I think, for £12 - they have the zips and under arm poppers, Asda and Tesco ones are about the same price but don't.
NCT sales. Small ads in local papers. Search Ebay your area - lots of bigger items, e.g. prams and cribs are pick up only, so far fewer people bidding on them and some real bargains to be had!
Check out the freebies with the pregnancy and baby mags - often worth more than the cost of the mag - again, only if the giveaway is something you want/need.
Watch out for sales - M&S one was good a little while ago, BabyGap is good too - a bit more pricey than the supermarket basics, but some nice stuff.