Bike shops take notice: if you want a customer to be thrilled with a run of the mill tuneup, the first thing you need to do is cut all of their cables. Seriously. Replacing a bike's cables and housing, especially if they're a couple of years old, has to be the best way to make brakes and shifters feel fantastic. Slick new cables and housing with intact liners go a long way to keeping indexed shifters indexing properly and drivetrain noises to a minimum. Besides, stripping the cables off and dropping the wheels makes it that much easier to give the bike a good clean- rarely a bad idea. Knowing this, I pay particular attention to my shift cables, especially on the mountain bike, where grit and moisture can quickly tear up housing's inner liner, especially when combined with the movement caused by full suspension. Because the decline in shift quality (from new) is often so slow as to be imperceptible and the improvement that comes from new cables & housing is impressive.
The last several times I've replaced my cables, I've used Shimano's pre-packaged XTR cable sets. While most shops carry large spools of bulk housing (usually sold by the foot), loose cables and cheap ferrules, Shimano's cable sets come with just about about the right length of housing (for hardtails- more on this later), a couple of slick-coated inner cables, and most importantly a full set of ferrules and boots. The amount of attention that Shimano has paid to their top-level off road cable set is actually pretty impressive- someone over there clearly understands the cables' role in good shifting. The XTR set is distinct from their Dura Ace road cable set: The housing itself is top-notch and each length is injected with a small blob of lime-green grease at the end nearest the logo. The endmost ferrules contain captive o-ring seals to keep crud out and each intermediate ferrules feature little straws that stick out of the cable stop and onto which a little boots pop (see pictures). The boot-on-straw setup does a very good job at keeping crap out of the housings without creating as much drag as o-rings likely would. The inner cable itself is smooth to begin with (cheap cables often feel rough when slid between the fingers) and coated with some slick stuff that holds up much better than others' Teflon coating, also helping to keep friction to a minimum.
With a decent (good) pair of bicycle-specific cable cutters (or a Dremel cut off wheel), a reasonably-skilled home mechanic can install the cable set within ten or twenty minutes, no problem. Once the housing stretches are cut to length, the cable should be fed through each length starting at the logo'd end (don't forget to install the rubber boots), spreading the grease along the cable's length. Because the intermediate housing section is a bit short for most full-suspension bikes, it's not a bad idea to pick up a couple of extra feet of Shimano's bulk 4mm housing at the same time as the set- use the XTR ferrules and you'll be in good shape. Fasten the cable pinch bolt and adjust the dérailleurs as normal. Do not touch the little screws. Do not leave the inner cable or housing stretches untrimmed. Your mechanic will revoke your home maintenance authorization if you do.
Not only are Shimano's XTR cables, when new, the best I've used since Gore stopped making their Ride-On cable systems (which have recently been re-introduced), but they hold up very well over time. With occasional light lube (I like White Lightning because it doesn't attract crud), they'll work beautifully for a year or two, depending on where and how often they're ridden. The $25 price tag may seem a bit high, but you'll quickly get close to that number with several lengths of bulk cable and inners to match. No other $25 you can spend, though, is likely to make your bike feel as good.