Discourse on the Four Noble Truths

The 2 Extremes to avoid
1.
devotion to indulgence of pleasure in the objects of sensual desire
pursuit of sensual happiness in sense pleasures
addiction to indulgence of sense-pleasures
That which is devoted to sensual pleasure with reference to sensual objects
2.
devotion to self-torment
pursuit of self-mortification
addiction to self-mortification
that which is devoted to self-affliction

Vibhava-tanha (craving for non-becoming) is wishing & e.g.
Wanting not to become sick, old, ugly, foolish etc;
Hoping not to become dead, poor, despised etc;
Desiring not to become bored, unsatisfed, confused etc.
In general: Craving for future not-(be)coming of all unliked states!
It is commonly present & dominant in mental states such as:
Fear, anxiety, worry, feelings of insecurity & various concerns!
It is basicly the mirrored opposite of craving for becoming of all
liked states such as craving for becoming rich, young, beautiful etc.
They are thus equally effective as the primary Cause of Suffering!
See also:
http://What-Buddha-Said.net/drops/II/The_3_kinds_of_Craving.htm