Popularized by black youth in the 70s and 80s yo mama jokes have become a mainstay in the adolescent development process.
Yo mama so poor
Yo mama so stupid
The basis of a yo mama joke is that it is both disparaging and from the point of view of a 10 - 16 year old. You can often make these funnier by breaking out of this norm.
Another nice trick is to bring in elements of pop culture:
Yo mama so easy
Just remember that in the end every boy loves his mama.