Enhance the surface of your art work with printed textures using a variety of tools and materials. Almost any textured object that retains its shape when wet will do. Sponges are especially fun to play with because they come in so many different textures. Leaves are so varied in shape that they, too, can add interest to a boring area. I'm sure you can think of many other ways to print textures; try them all.
Experiment with different types of paints and surfaces. Watercolors, acrylics, inks, even oil paints can be used for printing. You will generally get a clearer print on a smooth surface, but canvas or textured watercolor papers will also work. I like illustration board, which is what I used for the sample.
Some objects will print with thick paints, others require the paint to be thinned. Sometimes you will press the object into the paint, then print the surface; other times you might apply paint directly to the object to make the print. Try different ways of printing each time you use this technique to be sure you get the effect you want. Use prints to add finishing details, such as textures on walls or trees, or print lightly in a background to suggest distant foliage behind your subject.
My favorite sponge is the one at right with the big holes. I call it a Swiss-cheese sponge, and I haven't been able to find them for years. They look exactly like Swiss cheese, with different size holes randomly placed around the surface. If you know of a source for these, please let me know.
Use this technique sparingly so your art work doesn't get blotchy-looking.
For more texture ideas see The New Creative Artist and New Creative Collage Techniques.