Responses from other plurkers will gain you karma.
Updating your profile (picture, location, birth day etc.) will gain you more karma
What will lower your karma:
Spamming other users will lower your karma
Inactivity for a long period will decrease your karma
Karma will be lowered if you request friendship and get rejected
Getting unfollowed by friends will lower your karma
A good rule of thumb is to plurk like you have something to say/share. Plurk spamming will only force your friends to unfollow you and trigger the negative karma effect.