Never encourage them

Positive heckling: the worst sort of heckling.

Good on you for trying!
We all have to start somewhere! (Before you have even started).
This is the best thing I've heard all second!
Don't feel bad! (Said for no reason at all).
Interesting!
Yay!
That must have been good for your self-esteem!
Don't listen to them! (No-one has said anything).
Give yourself a pat on the back!
(Breaks into applause as you reach the third line in a 30 line piece).

Encouragement is never something that should be encouraged, people.