Hey Wayne, no problems with the bike doc thing; the more people using efficient transportation the better!
For tuning your mountain bike: first I'll deal with the common misconception about stretched cables.... There is no such thing.
BUT... cable housing can wear, ferrules can seat and plastic parts can abrade. All of these small factors can create slack in a once tight system, making it seem like the braided steel cable has stretched. For all intents and purposes the fix can be the same... or not. the or not depends on the shape of the rest of the drive-train system (bikes are like permaculture; all the systems are interrelated).
Simple fix: If all housings are in good shape, clean and lubricated; if the shifters and derailleurs are clean, and straight, if pulleys are not worn or dry; and most importantly if your chain is not worn beyond 75%
then you have a simple cable tensioning job on your hands. I love these.
1. hang the nose of your saddle off something stable, better yet get a upright bike stand for home repairs (this is the best investment in bike tools you will ever make! I have a running joke that most bikes can be fixed with a flathead screwdriver, a hammer and a blowtorch... if you have a bike stand. )I'd recommend something like this or better
2. loosen off all the tension on front and rear shifters by dropping your chain to the smallest cogs on front chain-rings and back cassette/free hub.
3. physically get behind your rear shifter and eyeball the alignment between your pulley wheels (the small wheels attached to your rear derailleur) and the smallest cog.
4a. if they are aligned perpendicularly but offset
you can adjust this with the two limiting screws on your rear derailleur. These screws are there to limit the upwards and lowest swing of your derailuer... to stop loose tension from dropping the chain off your lowest cog and to stop high tension from climbing the chain up into your spokes. They are a bit fiddly, but start with the eye-crometer (look at it) and try to line the pulleys with the cogs.
4b. if they are not inline perpendicularly
then you have a bent derailleur hanger. The ghetto option is to just bend it back by hand... BE WARNED, I have seen welded hangers snap off of frames with this option even on soft steel frames. If you have a removable derailleur hanger then this is a part that is built to be replaced... time for a trip to the bike shop.
5. Once you have your bottom limit set, shift up onto your biggest rings; front and back. Now do the same limit adjustment as step 4, but for the top cog on your rear gears. Now even if the rest of your shifting is a garbled mess, your top and bottom gears on the back should run ok (so long as you are not cross chained).
6. Shift back down to your two smallest rings, front and back (cross chained
7. Now is when you can release the cable retaining screw on your rear derailleur and pull any obvious slack through. Re-tighten the screw.
8. Try shifting up one gear with your rear shifter. if it doesn't quite make it or skips about, DON'T adjust the cable again.
9. Use your barrel adjusters!
Sorry I must run to work... more on gear tuning in a bit!