Mountain Biking

Lets start with mountain biking and useful maintenance tips to keep your bicycle in top shape.

keep your bike clean. You can more easily spot oil leaks, bearing failures and other defects if your bike is clean.
regularly check your wheel bearings for tightness. At least every 6 months re-grease your axle bearings.
if you do it yourself get the tools. You'll need a Park brand or equivalent gear cassette tool to remove the rear gears off the axle (to gain access to the bearings on one side for re-greasing).
you'll also need a "chainwhip" (any brand) which is basically a bar with some chain on it, this is how you hold the gearset while turning the gear cassette tool above.
use quality bearing grease. Any good automotive grease will work, those that resist water are even better.
when removing the gear cassette be sure to soak and clean it in either an environmentally friendly solvent or use hot water and ZEP liquid orange cleaner (Home Depot) and a brush. When it's dry (use a towel or blow it dry) you may want to use some light silicone spray to protect it. As far as I know they are usually made from exotic aluminum alloys or plated or stainless so corrosion is typically not an issue.
if you have a Marzocchi front suspension like me (Z1 dropoff) you can download excellent pdf files which show an exploded parts diagram. Maintenance is easy. The shocks come apart with relative ease.
if you don't have friendly solvent cleaner use hot water to clean the grime out of the shock body when you have all the parts removed. Dirt tends to accumulate in the bottom of the shock body (it settles out of the hydraulic oil). Dirt inevitably gets past the dust and oil seals in the shocks.
the more you do shock maintenance the better. Never use the crappy oil you can buy in the bike store when refilling your reassembled shocks. Always us Bel-Ray brand HVI "racing suspension fluid". If it's good enough for a Suzuki racing dirtbike then it will work great in your bike shock. HVI oil does not attack plastic like some of the cheap (but expensive) stuff available in your bike shop. I got 1 litre of it at the nearby Suzuki motorcycle dealer for $17.95 CDN which is $2 more than the bike shop stuff but you get 3 or 4x as much.
if your local bike shop only has stupid idiot mechanics on the payroll then you can call Norco Bikes in Canada to get Marzocchi parts sent to your favourite bike shop, they can also fax you a parts diagram and all the part numbers for your Marzocchi shock (very helpful). Three shops I called didn't know what part I was talking about when I asked them to order the plastic valve inside the valve body in the bottom of the stanchion tube. Apparently these guys rebuild shocks but don't know what's inside'em. This doesn't inspire confidence.
Marzocchi fails to mention in their manuals that if you have hardly any compression and zero rebound dampening the little plastic valve inside the valve assembly in the bottom of the stanchion tube could be broken. It looks like a plastic washer with a raised inside edge. Apparently they used to make this out of aluminum but it cost to much to manufacture so they went to plastic... a few 10ft dropoffs and these are toast. Fortunately they are $4-4.50 each (one per leg). I was told to ask for the grey colour ones as they're stronger than the white ones. Other things to note... bushings are $60 CDN per leg and the oil seal or dust seal is just under $15 CDN each.
when your chain looks worn and ratty get a new one! You'll only damage your sprockets with a stretched chain. If your sprockets are damaged you'll need to replace those as well or your new chain will be worn out much quicker. This is a good rule taken from motorcycles.
check your tire air pressure regularly.
look for damage to the tread or sidewall of the tires, if you do a lot of jumping double check your rear tire especially with hardtail frames (no rear suspension) that take a pounding.
check for rim runout.... look at your brake calipers, if you have disc brakes just check the rim as you ride. If it goes from side to side then take your wheel to a bike shop to have it "trued".