When you come up with a new idea, review how you came up with it, maybe you can improve it, or do better next time, or learn how to come up with even more ideas.
Spend time thinking - not reading or studying or talking. To get away from my normal "background" I do much of my creative thinking while walking. I keep a notepad in my pocket to write down my ideas so they're not forgotten. The changing, more random, background helps trigger new ideas and new ways of looking at old ideas. And sometimes reminds me of older ideas I had forgotten.
Browsing the web, especially when I push myself to follow links rather than reading more intensively, also has some of the same effects, though it's not as effective as walking - but sometimes the weather is too bad for writing down any ideas I get, either raining or snowing.
You must write down your ideas or you WILL forget them.
Play with ideas, explore unusual or random juxtapositions of ideas. Exploit ambiguity and metaphors to open up new ideas or approaches. Look for **more** solutions or ideas that could lead to solutions. Try changing one of the rules or constraints and see what happens.
Avoid premature evaluation - ideas that don't work can still lead you to ones that can - but only if you keep exploring them.
I prefer "creativeness" to the conventional "creativity", because the latter is too often reified into something in itself. Creativeness (or creativity) is actually a description of the originality of choices that we make - in design and in making decisions.