Just for the record with Gap. I've been watching their pattern with sales for baby and toddler stuff for the last 12 months. They do discount codes almost weekly (more if you are if you are signed up to their newsletter). New stock comes in (always on their website on a Tuesday) and starts being discounted at about 6 weeks in (at the latest). It goes into their sale properly at about 10 weeks after it hits their website. Don't buy anything from them at full price! You can get 30% off just about everything if you are patient. The except being 'seasonal' clothes (Halloween, Valentines, Christmas) which go on sale quicker. They had Halloween stuff reduced a couple of weeks ago!
They are definitely a company that uses discounts to confuse and encourage you to buy thinking you have got a good deal or that if you don't buy soon you will miss out when the reality is they sell most stuff at 30% discounted from the initial price. There are a few good discount codes to be had on their website though - 30 or 40% off all stock with free delivery on all orders is generally the best discount code they give (usually discounts only apply to sale or full price stock and free delivery only applies to orders over £50).
If you really want a bargain from Gap, you need to trawl instore regularly on their sale rails as this is where you'll find the biggest discounts. However stock is more limited and you need to be careful as the website can often have the same items cheaper!
I've not been looking at the discounts the Book People offer pattern very long but it does appear that they have a similar sale policy. Getting a genuine good deal from them seems to be more difficult than it might initially appear. Again, I think that its one to watch prices/discounts for a while so you can work out what really is a good one. My suspicion is you need to look out for flash sales and special offers in addition to discount codes with them as they (unlike Gap) tend to let you 'stack' discounts.
I probably should try and track both Gap and theBookPeople for discounting properly to work out what their best deals are. Are their any other companies that do this? I might add them to my list for the future reference and report back to Mumsnet about the best deals. I love spreadsheets and stats!