Please consult your physician before attempting weight lifting, also, 
please train with another person who is more knowledgeable in weight lifting
so that he/she can show you the right form for the exercises.  Start
out slow and easy, do not attempt to use heavy weights until your body
is acustomed to the workout.

Weight lifting is a great way to train for mountain biking.  Mountain biking
is very demanding on the whole body, not just the legs.  By strengthening
the body, the rider can ensure a better ride, with faster speed, better 
control, and less fatigue.

I cannot recommend any specific workout, but I will out line the current 
workout that I have been doing and I have been receiving good results from
Remember that everybody's body react differently to different exercises,
please do a little customization on the routine and workout frequency
to suit your needs.  For more info, please refer to misc.fitness.weights.

There are a few common myths about weight lifting.  One is that by lifting
weights, someone might get so muscle bounded that they might not be able 
to have full range of motion, or he/she might be slow.  When you look
at a football player, he is very muscular, but no one is going to call Emmit 
Smith or Barry Sanders as slow.  Yes, increase muscularity does restrict
some motion, however, you can easily counter that effect by stretching 
before,  during and after weight lifting.  I stretch after my warm up, then 
stretch between sets and after the whole routine is done.  My flexibility 
has actually increased. Another myth is that weight lifting can cause 
earlier heart failure.  This isagain false.  Assuming your heart is in 
very good condition, you should not have too much trouble lifting 
weights, however, I do recommend you to consult your physician before 
starting weight lifting.  If you have problems during or  after weight 
lifting, stop and seek medical help immediately.  Some people
claim that weight lifting can make them too big and that is usually not 
desired for mountain biking.  This might be true to some people, however, 
unless you are genetically gifted, you will not find yourself looking 
like Mr. Universe after a few months of lifting.  If you do, please 
consider switching sports to bodybuilding.

OK, the workout that I follow is called HIT, or High Intensity Training.  This
is a very high intensity workout that is done for very short duration of time.
Instead of doing multi-sets of an exercise, I only perform a few sets, 
however, that set is done to ultra high muscle intensity.  For example, 
when I do leg presses, I first warm up, I then put on about 85% of my 
maximum leg press weight on the machine.  I then proceed to perform as 
many strict repetitons(reps) as I can.  When I cannot continue to do 
more, I quickly remove a  little weight and go back and do more.  I keep 
doing this until I go down to about 50% of max.  When you do all this 
without any real rest, you will be extremely tired.  For a more detailed 
explanation, please refer to the HIT FAQ.  I only perform the below 
workout twice a week.  All exercises are done without rests in between.

The exercises I perform in my workout are the following:
Stationary cycling for 10 minutes 
light push ups 20x
light lat pull down 20x
the above 3 exercises are done to warm up the body.
Leg Press --1 set of strip set, from 85% of max down to 50%
Leg Extensions--1 set of pyramid up, from 50% to 100%
I start by doing as many reps as I can with 50% weight, I then have my
partner add more plates, and then proceed to do more without more than
20 seconds of rest.  I keep doing this until I cannot lift anymore.
Stiff legged dead lift--1 set of 12x
Calf raises -- 1 set of 20x
Bench Press -- 1 set of 12
Lat Pull down -- 1 set of 12
Bench Press -- 1 set of 8
Lat Pull down -- 1 set of 8
The above 4 exercises are done back to back without rest.  I usually use 
very heavy weights so that I need assistance when I'm at the 11 reps and 
I have  to force myself to do the 12th.  Please do not use force reps 
until you know what it is and how it will work for your body(again, 
check the HIT FAQ).
Military press -- 1 set of 8
Upright Rows -- 1 set of 12
Tricep press down -- 1 set of 8
Barbell Curls -- 1 set of 8
Wrist curls -- 1 set of 8
Cruches -- maximum.

All exercises are performed to positive failure, meaning until I cannot 
lift the weight up an inch.  This is a very difficult routine for most 
people, so please understand your limits and act accordingly.  

I cannot stress this enough:THIS IS MY ROUTINE, THIS IS NOT FOR EVERYONE!
Check with your physician before starting.  Also, READ THE DISCLAIMER AGAIN!