This seems to be the holy grail for UK writers. As a small press author who seems to have managed it in a few stores, speaking to my publisher I found out it isn't as easy as you think.
Some articles make it sound like walking into any random store with your book and waving it at the manager stand you in good stead of shelf space. This is misleading. Although Waterstones managers do have some discretion over what goes on the shelves, they do also have a central list that they pick from. The three basic things you need to get into a book store seem to be:
1) ISBN: you have to have one.
2) (in the UK) be listed on Nielsen
3) A recognised distributor who manages sale or return.
4) Access to the Waterstones independant books listing
These will let you get listed on the Waterstones computer system, without which the managers can't order the book. Now they can order the book, you need to persuade them to do so. If you have a good distributor, they will handle this, but otherwise you need
3) A good commercial, marketable, book.
4) A media pack of press releases,posters, and details of how to order your book
5) Good walking shoes.
Just because your book is available for them to order doesn't mean they will order it, or even know about it. You need to speak to the store owners, arrange events and promote your book.
Sales reps do this nationwide and have many years experience. Normally I'd say leave it to them, with one exception: if you are a compulsive bookbuyer and on first name terms with your local, (e.g. your store knows you well enough to special-order books before you even know they are out, phone you, and then fetch them out of the warehouse shipment for you) you might want to approach those stores yourself. It worked for me.
Here's some more information, straight from the horse's mouth:
(And if you read that, you can probably guess where one copy of my book and its covering letter will be going on 2nd December.)
One thing that surprised me: Being listed on their website for order is not the same as being available through their store system. If you are on Neilsen's feed, you'll probably show up on their site since they can order the book. Fire Season showed up there before it even had a cover, but was not available to stores for several more weeks.