Friendly insults are usually similar to real insults, but changed slightly to add that friendly feel. Take the word "jerk" for instance. It is curt and harsh, and can inspire feelings of hatred and malice. But, add the suffix weed to it, and it becomes "jerkweed". There's an insult that inspires feelings of brotherly love, peace, and harmony. There's quite a few modified insults out there. Another personal favorite is taking the angry "Sucker" insult, and changing it to the happy-go-lucky "Suckahfish" friendly insult.
Now that you've got some of the classics down, let's move on to the slightly more advanced friendly insults. These are the ones you make yourself, using some key words. The most important of these is "face". Face, by definition, is a term of endearment (note: It's somewhat possible that definition was made up by a friend). So by adding face to the end of nearly any insult, you immediately lighten it up and change it to a friendly insult. See how nice it sounds when you change lame to lame-o-face? The o isn't usually needed when you add face, it just makes this one flow better. Hopefully this has been educational for you. There are many more rules, but hey, let's just start with the basics. Now you're equipped to strengthen any friendship.