Good suspension setup is very important. Read the owner's manual and reset all the clickers to the specified settings. I am lighter weight so I go for the "comfort" setting. Make sure you reset all the clickers properly - the previous owner may have them set for something else. Also, when you buy a used bike, try to find out if the suspension was altered for a lighter or heavier rider.
Depending on the motorcycle, we can customize the suspension a lot. Competition bikes have the more expensive suspension systems, offering the most adjustabilty:
The initial setup should be done at a reputable shop. There are many ways to adjust the suspension to suit your body and riding style and we can bring it into the ballpark initially by customizing it for your weight and type of riding you do:
Front / forks:
You can often just take the forks off and take them to the shop, give them your weight and talk about the riding you expect to do: motocross racing, enduro or both.
Next is the sag. The front and rear sag setting should be different from bike to bike, depending on suspension travel, spring stiffness and type (progressive or not). It is best to follow the owner's manual for this. It is a very important setting, which takes some work, but changes the behavior of the bike a lot.
The sag is changing the springs (which you did already) and then setting the preload: how much are the springs pre-loaded or pre-compressed before you even sit on the bike - so the sag is the difference between an unloaded bike (wheel hanging) and with you sitting on it.
The exact numbers and way to set it differ slightly from bike to bike - see the manual.
The term "clickers" refers to the screws and nuts used to adjust damping, front and back, - the proper adjustment is based on the number of "clicks" (or "turns" for some adjustments).
Usually for woods riding, you set the damping to be softer and make it harder for motocross. If you alternate between the two, either play with the clickers every time or find a compromise.
The front forks have compression damping (which applies on the compression stroke, when compressing the forks) and rebound damping (which controls the rebound of the forks, after they were compressed, as the wheel is pushed back.
The damping is changed usually by screws (or knobs in some forks). Regularly, the compression damping screw is at the top and the rebound is a small screw at the bottom of the forks.
The damping can be hard at one end (screws in all the way) or soft at the other end: screws out all the way.
The compression damping applies on the "compression stroke" so when the bike is landing a jump, you get hard on the brakes or you go over rough terrain. A soft setting which would be awesome to smooth out rough terrain will not work well if you're landing high jumps and will also make the bike wallow a lot when you get on the brakes - so the trick here is to nail the right compromise.
If you jump a lot (i.e. motocross) the damping should be harder. If you go enduro, then a softer damping will smooth out the terrain.
The rebound damping controls the rebound stroke, as the suspesion is extending. A soft setting here (slow) will cause the wheels to loose traction sometimes, as they loose contact with the ground. A hard (fast) damping here will allow the wheel to not loose contact with the ground.
The rear shock, supplementally, has settings for "low speed damping" and "high speed damping" damping, so you can adjust both separately.
Do follow this process and save yourself some hassle. This setting can change the bike's behavior significantly.
Always start with, from the manual, the normal settings for the bike. Especially when you buy a bike, make sure to reset the damping to the manual specs and work from there.
You can make adjustments in either direction from here and note how the bike behaves. It's important to take notes: how many clicks in/out and how does the bike behave. Always consult your notes and keep adjusting until you like it. If you're lost or you don't like it anymore, always go back to the manual default setting and start again from there.
It is not uncommon to have the damping changed slightly with conditions of the next track or ride... so carrying your notes with you is important.
Other dirt bike setup topics: