It's a slippery slope, keeping a bike in good working order. Whenever I've taken charge of a brand new bike, I've generally found it to be maintenance-free for a good year or more, except for tyres and tubes and the occasional chain lube, and then it suddenly seems to have had enough. Nothing major, but brakes, cables, chain, jockey wheels, sprockets, chainrings and handlebar tape all seem to lose their lustre and/or performance in rapid succession. If I'm unlucky then the wheels and bottom bracket can also start to show their age. A complete overhaul is then necessary, and then my 'new' bike is definitely not 'new' any more!
This situation is largely due to laziness; a quick clean with soapy water every few months really isn't sufficient, and it is very important to identify and service the awkward parts that escape a quick scrub.
Servicing Aids
Component Recommendations
Good Retailers
The best retailer is of course your local bike shop, if they're up to snuff. They will not only sell you the right bits, they will also advise you before and after the purchase, and will usually be inclined to help you out when things go wrong.
Of course, if you know exactly what you want and exactly what to do with it, and the price is right, then go mail-order. Wiggle has generally the best prices as it offers no advice and seems to use as little packing material as it can get away with. They usually advertise in the Comic (Cycling Weekly) and Cycle (the Cyclist Touring Club's magazine).
One I've started using recently is Chain Reaction Cycles with its UK operations in Ballyclare, Co. Antrim. You can also call them on 028 93352976 (but not at the weekend).