All the technical painting skills in the world will do you no good if you haven't got ideas for paintings. So where do you find ideas you can use to create and develop your own, distinctive paintings? Here are the options and approaches I believe in.
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 idea as a starting point, not the end point.
You can't have ideas for paintings without having an idea of what style of painting you want to make, or what genre. So the first step to finding painting ideas is to make a list of what you options you want to consider.
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 Ideas Down on Paper, in a Sketchbook or Journal
Don't be misled or intimidated by the pages you see reproduced from sketchbooks where everything is immaculately executed, with every page a perfect sketch. A sketchbook is a working tool for ideas and record keeping, not a work for display. What you put in it and how you do it is entirely personal, like a diary.
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.
While I love to travel to new and favorite locations, the place to start gathering ideas is where you are right now. Your living room and kitchen will provide props for a still life. A garden will provide plants and flowers that change with the seasons. A scenic viewpoint will provide a landscape or cityscape that changes with the time of day. Persuade family members to pose for you, or sketch passer's by from a coffee shop. Paint the family cat or dog when its asleep. Take photographs to use as reference if you can't spend much time at a location.
There's no rule that says you can use an idea only once. On the contrary, a painting idea can be used to create a whole series. Take an old painting you like and work on variations, pushing the idea around and further e.g. different color sets, different angles, different lighting. Just look at what Monet did with his haystack paintings.
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."
Ask other people for ideas, you never know what they might come up with, and look at the work of other painters (both living and dead). Make notes of paintings that inspired you, or caught your attention. Create your own versions of other people's paintings (with an acknowledgment of the source) as a starting point, then push the idea further.
The Painting Ideas Machine contains a collection of ideas from numerous people, 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.
Don't ignore rich heritage and sources of ideas from past centuries of painting. If you got put off art history by a college course you found boring, or think it's something too academic to be interesting, then approach the past through artist's biographies or TV documentaries and films instead. It's not the subject that's boring, it's how it's written or approached that makes it interesting (or boring). If you've never read any painting history, I'd start with Simon Schama.
Instead of changing your ideas, change what you're using to paint those ideas. Try a new medium, or a combination of mediums (aka mixed media) to free up your brain from automatic and jaded painting styles. Stop reaching for your favorite paintbrush and putting the paint on the paper in exactly the same way that you find comforting and easy. Stop using your favorite colors and try some new combinations.
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.