I just wrote a tutorial about self-watering (mainly for indoor plants, but will work for some outdoor setups): really self-regulating, and no need for electricity, pumps or timers.
It is here: http://www.instructables.com/id/Self-regulating-watering-system
Hope someone find it useful.
DIY Self-watering system
If the tube tneds to block with air bubbles, you might try thicker tubing with something absorbant pushed through the tube. The absorbant material would act like a wick even if a bubble occured.
a strip of cotton flannel?
I never saw yarn that was water-absorbant, but if there is, that might work.
Thanks for the idea.