Dog aggression is very common in all of the Spitz dog breeds as well as some other families. Aggression is probably the most misunderstood trait of any canine. Aggression is commonly dealt with in the wrong way, as many dogs show aggression as a sign of dominance over an not-so-confident owner or intruder.
When you make quick movements, your dog may be startled, and he/she could nip or claw at you. This can understandably startle you, causing you to draw back, which teaches your dog that aggression is ok. Though aggression is totally natural in dogs, it is our responsibility as owners to teach them that it's not ok to be aggressive toward people or other animals.
One of the most common ways to stop this behavior is with a "negative mark", which is a behavior that your dog learns to associate with negative opinion. Put simply, it means "don't do that". These are usually actions that startle your dog or interrupt the behavior while it's being performed. The most widely used of these is the classic "Bad Dog!", but others are "No!" or "Stop That". Feel free to get creative as long as it gets the point across.
Actions such as loud claps also work well for interrupting bad behaviors, as they can startle your dog and cause him/her to associate the behavior with the loud, undesired noise.
Positive reinforcement for doing the correct behaviors in situations with negative stimuli are also a definite must to reinforce your training efforts and keep your dog on his/her toes (er...paws).
These can be treats or petting or even a quick trip outside to reward him/her. Whatever you do, make sure to always reward your pooch for good behavior, as this is just as important as stopping bad behavior.
With aggression, one of the biggest things to remember is to not withdraw when the situation arises, but never offend your pooch either. This can cause even deeper issues than simple aggression.
As with any training method, the situation dictates your exact actions, but these guidelines should aid in dealing with aggression and other undesired doggie habits.