We have been on Wonder twice and Magic twice, booked again for Magic next year. See my list below for dates we have been.
1st 4 we have booked direct with
DCL on their website, very easy to use. The next one for August 09 booked thru Dreams Unlimted, they are usually the same price(sometimes slightly cheaper, never for me) but offer onboard credits, we got $100.
You must book as early as poss, over a year in advance if you can, then you get best price, pick of cabins and choice of dining times. We always want a secret porthole room, they only have a few on each sailing but you pay for inside cabin and get outside with restricted view, they are great value. If the price is reduced after you book you pay the lower price, booking with DCL you have to check prices often, DU check the prices for you.
Before you get onboard decide if there is any chance of you doing another as booking onboard is the cheapest option, last month we saved $452 just for 5 minutes at the rebooking desk, then transferred the booking to DU when we got home.
You can book in UK with companies like TCD, DONT, they are way OTT, I helped someone recently cancel their booking and go with DU, it saved them over £1,000

thankfully didn't cost anything to cancel.
Prices vary a lot for different sailings. Next year we are doing 7 nights in August cheaper than I can get 4 nights this August, we have paid as little as £50pppn another I priced up was £140pppn(New year, too much for me)
Book before the 28th May when fuel surcharge is added if you can, It will add about £4pppn see here
http://www.wdwinfo.com/wdwinfo/cruise-new/news.htm#toystory
Good luck, ask if you need anymore help.