Killing/POPing a Unibear will instantly bring your multiplier instantly up to 100X.
Your multiplier decreases over time. You can use Freeze Capsules to freeze your multiplier for a couple seconds. This will not freeze it when it's going up, however. Freeze Capsules are found on the ground near bridges and woods.
Bonus points are not affected by the multiplier. They're pure points.
To keep a multiplier up for a while, you need to create a strain of naughtiness. Wear a bear costume (or any type of bear that appears in a level). Then start attacking other bears. This will start a faction war. As the other bears attack eachother, your multiplier will go up a bit for a while. Dont waste time though. Go do the things you have to do to get the points. Mainly, start with the items you have to destroy. Destroying items with MAX multiplier will bring MAX points.