Sounds like you have a couple things going on. 1st I'm sure those springs are way sacked out they don't last long. 2nd if the shocks haven't been serviced lately they could be a big part of the problem.

I don't have any measurement specs for you to go by, as I just kinda wing it when it comes to the sag in the suspension.  You generally set it up when new and then comare to that as the shocks and springs wear out on you.  If you've got the rear scissor springs set to maximum tension and the sled still drops alot when you sit on it the springs are probably sacked.  Mind you, there is supposed to be some sit in so the rear suspension can drop into holes in the trail as you go over them and then have the full suspension travel when coming out of said hole in trail.  And as was already mentioned, if the shocks are of the rebuildable type you'll want to have them rebuilt every 2,000 miles or every season.  This milage isn't set in stone, as some folks tend to ride their sleds harder than others.  Also some folks are much heavier than others, which will also play a roll in how often you rebuild shocks and replace springs.  If this '01 ZR has never had the shocks serviced or the springs replaced and it has anywhere near 5,000 miles on it then that suspension is in need of servicing.  The front skid shock will have an effect on ski pressure and not ride height.  If the shocks just need a rebuild/recharge, it's not that expensive.  mk2g60 (Eric) does a fantastic job rebuilding, recharging, and even revalving all kinds of shocks.  He's right in West Bethel, so if your close by you can drop them off yourself and save on the shipping costs.  I realize that this may look like we're trying to talk you into ashock rebuild that you think you may not need, but trust me on this.  You ride 2,000 miles a year and don't touch the shocks for two seasons and then have them rebuilt for the 3rd season and you'll think you just jumped on a brand new sled.  It really makes that much of a difference.

   I'm suspecting that the '01 ZR800 your talking about is the used sled that your friend just bought with 1,200ish  miles on it.  If that is indeed the case, wait for snow and ride it to determine what the suspension might need for adjutments and/or replacement parts.  I'm not sure of the regular ZR shocks are rebuildable or not.  
    I know for a fact that the Sno Pro shocks are rebuildable, but I don't know how to test them except that when I don't like how they ride anymore I have them rebuilt.  Eric would probably be able to tell you wether or not they need rebuilding.  What spring are you asking about?  Ski springs (coil over), front skid spring (coil over), or the rear scissor springs (torsion)?

   Let's start with the rear skid scissor springs.  These are set up to hold the riders weight given that riders particular riding style.  Stiffer springs = harsher ride with less bottoming out.  Stiffer rear scissor springs will also reduce weight transfer to the track and add more ski pressure.  Softer springs will obviously have the opposite effect.  These springs are adjustable, so before replacing them make sure that your using them to their siffest setting.  There is a triangular block on these springs with a built in nut.  Turn the nut to turn the block.  The taller part of the block on the springs will give you the stiffest setting and the opposiste for the shortest part of the block.  There is a block on both sides of the suspension, they do not have to be set in the same exact positions.  One can be on the stiffest and the other can be on medium ect..  They work together, so you end up with 5 possible settings. 1) both on the short part of blocks 2) one on the short part and one on the medium part of blocks 3) both on the meduim part of blocks 4) one on the medium part and one on the taller part of blocks 5) both on the taller part of blocks.  1) being the softest setting and 5) being the stiffest setting.
    Now onto the front skid coil over shock or center shock.  Increasing preload on this shock (tightening the spring) will give you less ski pressure and more traction on the track as you'll have more tack on the ground this way.  Loosen that same spring and you'll gain ski pressure while losing some track traction.  You'll also bottom out the suspension easier with the spring set soft as this is the first shock in the skid to hit the bump, but it has less of an effect on bottoming out than the rear scissor springs have.  Most folks don't mess with this spring as it can make a big difference in ski pressure.  The other adjustment in the front of the skid is the limiter straps.  Letting the straps out (more slack) will allow more track to the ground giving you more traction and less ski pressure when on the throttle.  When getting off the throttle the front skid shock takes over and will determine how much ski pressure your going to have.  You have to picture the rear skid rocking back and forth when on and off the throttle to see the effects of these adjustments.  
    On the ski shocks increasing preload (tightening springs) will give you more ski pressure and visa-versa.  Also increasing preload will be more beneficial to reduce bottoming out up front.  The more preload you have will also lift the front of the sled a bit, which will in turn reduce how much track is on the ground.  
    All of these adjustments are give and take.  Make only one adjustment at a time to see the difference before going any further or you'll end up chasing your tail trying to get the handling right.  If you onoly make one adjustment at a time and the adjustment makes the handling worse you'll know exactly what to adjust back and not have to guess which adjustment made the handling go south.  
    May I ask where you live?  This could be explained much easier on the trail.

i live in ipswich mass and ride out of rumford and andover ...thanks for the info....i cant wait to try it out on the snow.......lets get the snowdance going!!!

Quote from jaggede on 11/30/09 at 11:03am:

 
 
I've ridden out of the Andover Guest House a few times, good trails up that way.

FYI: I used to have a 1998 ZR 600 (not a sno pro) and I had the shocks recharged. White Rock Outboard in No. Gorham will recharge them for $35.00 a piece.

Yes they were.
I don't know if they can do others since they are a Cat dealer.