I find I come up with ideas best when I'm not actively working on the problem. Or that is when ideas come to me. The key is to start sketching and brainstorm any ideas you have, good or bad. Once you have some initial ideas down, if you get stuck it can be helpful to step away from your work. Go for a walk. The different environment may jog your brain. I find I get a lot of ideas when I am walking. I don't know if it is the movement that helps.
Another thing I do is work in coffee shops when I am in the concept stage of a project. This puts me in more of a work mode, as I am out of the house. I go to my local coffee shop and work there for a few hours. I find I am more productive when I do this. I also keep a notepad by my bed to jot down any ideas. Sometimes when I stop actively working on a concept and let my brain relax (such as before bed) ideas will come into my brain. Usually this only happens if I have already started working on the project. My brain continues working subconsciously on the project.
Having a relaxing bath or shower also helps as it lets your brain relax and subconsciously come up with ideas. Basically I find my best ideas happen when I am not working. My process is work, relax, jot down ideas, continue working. I also find sleeping on ideas helps, as my brain works on the problem while I am sleeping, and in the morning I have new ideas.