ahhh I love Crufts! My favourite day of the year
Plenty of good advice given - COMFY shoes and practical clothing, plenty of cash because most of the stalls do not take card payments. There are cash machines dotted about but they are not free to use and charge a princely sum, so get your money out beforehand. I'd also highly recommend looking on the Crufts website for a list of trade stands and make yourself a shopping list. Be very careful of presuming that everything will be cheaper at Crufts because it isn't always - as smokeybear said, you can always use Crufts to look at the product in real life and then order it online afterwards. If it's a product that several trade stands are likely to sell (say something like a Thundershirt) then take a notepad and pen, or make a note on your mobile phone of which trade stands are selling it and how much for - you can save a few quid by shopping round within the trade stands, so try not to buy from the first place you look - but DO make note of which stalls you want to go back to and where they were because otherwise you'll forget and get lost as Crufts is unbelievably HUGE!
I'd also look on the website and look at what displays/activities are on the day you are there - if you want to watch Heelwork to Music or Agility, you can go and sit in the arena (for free, for as long as you want!) and watch, I usually take a packed lunch and sit and eat whilst watching the agility
Also if you want to watch any specific breed judging, find out what ring they're in and what time they are being judged, it's a real bummer when you want to watch some judging and miss it! (as I did with the Borzois last year because I couldn't find them!) If you don't know where a certain breed is being judged, just ask someone who has that breed with them, they will know what ring/hall they're in, and if you can't see anyone then just ask any exhibitor (who doesn't look too busy!) if they can have a look in their catalogue for you and find out.
Definitely take your own packed lunch with you, food there is extortionate and you'll have to queue for ages, it's much better to take your own butties and a drink - a bottle rather than a can, then you can drink it throughout the day as it gets hot and tiring in there!
So to sum it up - sensible clothes, plan what you want to see/do beforehand, make a shopping list, take your own food & cash! And most importantly - have an amazing time!
I love Crufts, 2014 will be my 7th year there!