I also think it's crucial to allow time to experiment. Be gentle on yourself and allow yourself to make mistakes, to go down dead-ends, to see what might develop. Use each of these painting ideas as a starting point, not the end point.
1. List Your Options, Your Likes and Dislikes
What subjects / styles do you think you'd like to make (also list what you know you don't want to do), then narrow it down from there. For example, do you want to paint figures, landscapes, abstractions ...? What style do you want to use: realistic, expressionist, abstracted ...? Are you going to use a limited palette, or have one color dominate?
Too many options is as paralyzing as too few, so narrow your list down to one or two and start working with those. Use these printable art journal pages to get going.
2. Put Painting Ideas Down on Paper, in a Sketchbook or Journal
I use a sketchbook more like a creativity journal, with as many words as pictures. I have a pocket sketchbook and pen with me most of the time, and a larger one for when I'm painting on location. I don't worry about being neat or organized, I'm merely recording thoughts and ideas for possible use on the proverbial rainy day.
See: Keeping a Painting Creativity Journal and Sketching: Is there a Right/Wrong Way?
3. Gather Painting Ideas from the World You Live In
See: Tips on Painting from Reference Photographs and Reference Photographs for Artists.
To quote that motivational book that's such a favorite of mine, Art & Fear (on page 56): "One of the best kept secrets of artmaking is that new ideas come into play far less frequently than practical ideas -- ideas that can be re-used for a thousand variations, supplying the framework for a whole body of work rather than a single piece."
See: Painting a Series called "Heat" and Creating a Body of Work
5. Ask Other People for Painting Ideas
The Painting Ideas Machine contains a collection of ideas and will randomly generate a suggestion at the click of the button. Approach it with an open mind and give each idea some thought for where it might lead. Dismissing multiple ideas with only a moment's consideration is a loser's approach.
6. Expand Your Knowledge of Painting History
See: Great Art Reads for Painters and Books for Creative Inspiration and Ideas for Artists. (And don't forget About.com's Art History site.)
7. Get Off Auto-Pilot and Try Ideas in a Different Medium
Make a huge switch by trying something such as watercolor pencils and a water brush, or encaustic painting. Or if you're used to working with wet color, try working with dry color in the form of pastels. Or add a medium to speed up or retard the rate at which your acrylic or oil paint dries.
Daily Art Ideas (including photo galleries for sharing your paintings)
31 Painting a Day Ideas
28 Painting a Day Ideas
31 Painting Ideas from Poetry
30 Painting Ideas from Idioms and Sayings
31 More Painting Ideas from Idioms
30 Painting Ideas from Classic Novels
Enjoy using a photo to jumpstart a painting? Join these regular challenges to create a painting using the reference photo provided, in whatever style you choose. Subject range from a sunflower to a castle.