For our suspension to be able to carry the weight of the motorcycle and the rider, we need to make sure that we have springs that are strong enough for that combined weight.
Most bikes are delivered from the manufacturer for a rider that is about 70-75kg. If that matches your weight, great! If not, it may be a good idea to talk to a suspension technician and figure out what springs you need.

A spring that is too soft will have your suspension working in the lower range of its stroke. If the bike is equipped with 100mm of suspension travel, then we would like to use the most of that travel and not have the suspension sitting at the lower 40mm because of soft springs. 
A spring that is too stiff might cause you to never use those bottom 40mm even though you are braking hard and jumping the big stuff. 

Setting the sag of the bike helps make sure that we use most of the suspension travel available and, most important of all. It sets the stance of the bike and that is what tells us if the bike will turn on a dime or if you need to force it to turn and it still moves slowly from left to right.

We are aiming for a sag, front and rear, that is somewhat equal. Meaning that if we set the rider sag at 30mm in the front, we are looking to get 30mm in the rear aswell.

So how do we measure sag?

Sag is a combination of three numbers. Unloaded, free sag and rider sag.

Unloaded is the bike with the wheels in the air. The suspension is fully extended and we will take a measurement from a fixed point on the swingarm/front axle to a fixed point on the bike. Why not use a marker and make sure that you measure those points every time.

Free sag is what we measure when the bike is standing on the ground with no rider on it. The pro version of doing this is the push the bike down and let it return to its resting state, take that measurement. Then lift the bike up and let it come down to its resting state. We do this to eliminate stiction.
The middle point between those measurements is your free sag.

Rider sag is when you sit on the bike, with all of your equipment on you. We want you to be the same wheight that you are when you are riding the bike. Try and sit in the same spot that you do when you ride. Hands on the handlebars.

Depending on what kind of riding you are doing there are different recomended sag numbers to aim for.

Supermoto
Free sag  ~5-10% of total suspension travel (~14-28mm)
Rider sag ~20-30% of total suspension travel (~45-70mm)

MX
Free sag  ~10-20% of total suspension travel (~20-35mm)
Rider sag ~30-45% of total suspension travel (~75-95mm)

Now, these are recomendations and they might not be the right numbers for you. Use them as a base and adjust from there. 
The preload on the rear shock is normally at little bit easier to adjust then the ones in your fork. This means that you can use that preload to make smaller adjustments to the stance/attitude on your chassie. Add a bit of preload to help the bike turn in faster and reduce it to slo it down and make it more stable.

If you feel that the bike starts to missbehave of if you need to add a lot of preload in order to get the bike to turn as you would like, try lowering the front of the motorcycle by loosening the triple clamp and sliding the fork legs up throught them. We would recomenc doing it at 3mm at a time to gauge and see if you like the behaviour you are getting. Or maybe it´s a good idea to start looking at getting one of those adjustable pull-rods for your motorcycle?