Spring preload is normally adjustable by the spring preload collar on rear shocks, or via the top cap adjuster on forks.
These adjusters compress the spring and should be used to fine-tune the correct amount of sag. Turning the spring preload collar to compress the spring will decrease the amount of sag, and vice versa. They will increase or decrease the hardness of the suspension slightly but should be used with care or damage can occur. Too little preload and the spring will rattle and possibly damage surrounding components, too much and it's liable to snap.
On front forks, turning the adjusters will usually increase or decrease the amount of spring preload by between 5-10mm or 0.2-0.4". So if you had a 60lb springrate you would increase the initial force required to compress the fork by 12-24 lbs and similarly the ending spring rate would increase by the same amount. This may seem like a large increase and therefore many people think adjusting the preload will make a huge difference but in reality it doesn't. So if the fork was a 5" travel mode the ending spring rate would increase from 300lbs to 312lbs or 324lbs, which is only a 4-8% increase. If however you were to go to the next spring rate, say 70 lbs, you get an increase of 14% and the ending rate would be 350lbs or with preload 364 to 378lbs.
On rear shocks, the spring preload collar is infinitely adjustable (with a little help from some pipegrips!) so in theory you could get the ending rate you want. The only problem here is the limited space for the spring. As you screw the preload collar down to compress the spring, the gaps between the coils become smaller. These gaps must add up in total to a value higher than that of the shock stroke. Ideally they should be 10% greater otherwise premature failure of the spring will occur. This is why all coil-over shocks have a maximum amount of allowable preload. This amount is usually between 2-8 turns and is counted from the moment the spring collar contacts the spring and begins to compress it.
As most suspension is supplied for Joe Average who is 170lbs or 70kg you will usually need to take different springs if you are more than 10% ether side (assuming the manufacturer called the spring rate right in the first place!) if you want the ultimate ride.