Here are some things I use....
Lift some of the blue from the sky to have the effects of clouds by blotting
with paper towel wadded up. This is also good to do when painting a dirt
road or path.
I've not done this, but I've seen students step on wet paint with tennis
shoes that have "great patterns".
Thin paint and pour it on top of dry paint.
Press objects with texture onto wet paint
Wad up plastic wrap and dab wet paint, scratch into it with handle of brush
or plastic knife.
Use small pieces of mat board to "paint", by scraping first one way and then
another...drag or dab edges of board to make lines.
Use mat board pieces to "scrape away" paint.
Spray with water or alcohol while wet.
Mix sand with the paint...or pour glue down and sprinkle sand in it...let
dry and paint over.
Draw over dry paint with crayon, pastels, or oil pastels.
Some of these techniques work better with tempera and acrylic and some work
better with water color.
E. B. Frink Middle School