I think the key to success in sales (not just bikes but anything really) is to be super organised. Narrow down your target bike to 1-3 options. Know the exact spec you require and the size (which will probably be different for each manufacturer). Be clear upfront on any compromises you are and are not willing to make and don't deviate. Know what your ceiling price is - what you can afford and what would represent value for money for you. Again, resist the temptation to deviate.
Once you have that list, identify the retailers that sell those bikes. Keep tabs open in Google Chrome for each of them at all times. Sign up for their email newsletters and check each website at least once a day. As soon as a bike that fits the criteria comes up, hit the buy button immediately.
I bought my London Rd this way. I narrowed it down to 2 bikes, knew the specs and prices I wanted to pay. As soon as the email alert came through with the sale deal, I'd bought the bike within 2 minutes. There were only 2 available and the discount was huge. Planet X have tons of sales but I later found out that the reason for this one was that they were introducing a new spec and flogging off the older version. There are loads of reasons for sales and the traditional end of the cycle season is just one of those reasons. Shops might want to get rid of a bike that has been sitting around on the shop floor for months, there might be a slight scratch or other defect, they ordered in a bike for a customer who then didn't want it, they have an odd size they can't shift etc. etc.
Good luck with the search. I look forward to hearing what you get.