You should do your jump training at the conclusion of your match,
practice, playing, whatever so that you can train to failure without
interfering with you performance. When I did my training, it consisted
of several sets of different types of jumps all used in volleyball.
Thus, you can work on your footwork as well as your vertical. I start
with block jumps with four different sets of at least 8 repetitions
(use an equal number for all four). The four sets were: two-step to
the right, tw-s to the left, three-step to the right, th-s to the
left. This can be done with or without a net (just imagine one if
necessary). Concentrate on perfect technique and always use your max
jump. Do not rush through the sets - you are not trying to get
cardiovascular
conditioning. You want your legs to wear out first, not your
cardiopulmonary system. 

Next, do sets of at least 12 of three different kinds of approach
jumps - left side, right side, and middle (i.e. jump straight up and
do not glide). One option is to make reps 6-9 be backrow approaches -
thus working on broad jumping as well. Again, always use perfect form
and your max jump. 

The final set is the toughest. It consists of jumping straight up off
of two feet using your max jump repetitively. You do this until
failure. I don't mean failure as in until you die, but as in until you
cannot really jump that well anymore. I used to do this by standing
under a basketball rim and jumping up to touch it with both hands.
Failure consisted of not getting both hands on it three times in a
row. That ruled out accidental "misses" and whatnot. I could usually
get to about 50 or so. You can do this at the net just as easily as
with a rim. 

General info: I generally stuck with repetitions of 8 and 12 for the
different sets, and as my legs got into better shape I would switch
over to doing more sets, ie 2 times through the blocking sets, then 2
times through the approach sets, followed by ONLY ONE set of the death
march. 

You should be doing this AT MOST three times a week, otherwise you
will OVERTRAIN and do yourself more harm than good. Make sure that you
get at least one, preferably two days rest (no jump training - playing
is OK) between sessions. If you have access to sand, the sand will,
training in the sand will only help you marginally with your jump -
the jumping is no different than indoors, but moving through the sand
on blocks and approaches may help overall leg strength - but the sand
will be much kinder on your joints than the pine wood. 

You can optionally supplement the preceding jump training regimen by
replacing one day of jump training with one day of weight training.
Since it sounds as though you have done some weight training before I
will give you the following program, but exercise extreme caution - it
is for POWER training, and you should not do power training until you
have at least 6months-1year of consistent weight training on that body
part. 

When I train legs for jumping, I do the following exercises in this
order - Squats, leg extensions, leg curls, leg presses, calf raises.
All of the exercises should consist of a pyramid of sets which
consists of: warm-up, 10-12 reps, 7-9 reps, 4-6 reps, warm-down of
10-12 reps. Each set, except the warm-up should consist of as much
weight as you can handle and maintain proper form while performing the
specified number of repetitions. The warm-up should be a weight such
that you can perform 16 reps comfortably while maintaining perfect
form. All of the sets should be conducted to failure - when you can
accomplish more reps than the intervals specified, increase the
weight. On the warm-down perfect form should be the primary goal. 

Since the exercises consist of training until failure, an adequate
spotter
is essential. The proper technique on all of the exercises is
essential. For squats, that consists of keeping the back very straight
(not arhced either way) and the head up while bending the legs to the
point where flexion is just beyond 90 degrees with the feet slightly
broader than shoulder width. Bending the legs beyond
that increases the likelihood of immediate as well as long-term
injury, as well as not being useful to jumping since you do not bend
your knees beyond 90 degrees when jumping. Normal repetitions consist
of slowly lowering to the proper flexion and then extending back up,
NEVER locking the knees. The pace of the exercises should be slow and
steady. When training for POWER (SEE CAUTION ABOVE), the speed of the
negative
phase is unimportant (lowering of the weight), but the positive phase
should be EXPLOSIVE. You should come up with as much speed as possible
- as though you were jumping - WHILE MAINTAINING PERFECT FORM. You may
find that you need to slow the movement down near full extension in
order to avoid leaving the ground.

For all of the other exercises, the speed guidelines are the same, but
proper form differs. In leg extension, the flexion used should be that
limited by the machine, but never too far beyond 90 degrees, and it is
vital to avoid LOCKING the knee at full extension. Briefly pause at
full extension before lowering. Always keep both
feet with toes pointed and keep them approximately shoulder width
apart. In leg curls, exercise the same caution against leg
locking, and flex to the maximal position and hold briefly, then
lower. Again, keep toes pointed and feet at shoulder width. In leg
press, again only barely exceed 90 degrees of flexion and do not lock
the knees. Feet should be slightly wider than shoulder width. Do NOT
hold the weight at maximum flexion. In calf raises, have the feet
shouder width apart and start with the heel below the toes. Extend
until the heel is at the maximal position above the toes and pause
briefly before lowering until the heel is below the toes again.

General info: ALWAYS use a spotter. ALWAYS use PROPER technique. DO
NOT POWER TRAIN until you have been weight training for at least 6 mos
to a year on that body part. ALWAYS, ALWAYS, ALWAYS, warm up with at
least five minutes of light CV work (running, biking, rowing,
stairmaster, etc) before starting your workout. Make sure to STRETCH
the appropriate muscle group before each exercise. 

As always, nutrition and supplementation can enhance any benefits you
might gain form the above, but that's another story.

The combination of the above, repeated on a regular basis took me from
a 28" vertical when I graduated from high school to a 40"+ vertical
and a place on the National Team. I highly recommend them, but be
careful not to OVERTRAIN - the cardinal sin of any kind of training -
one of which I have been guilty on more than one occasion. If you
chose, do only the former for a long time. It was enough to get me
from 28" to 37" in 6 months. The latter was what pushed me over the
top, but I only did that once I got to the higher levels and needed
something more. Start slowly and build.

-marin gjaja