Keep a Journal
Mae West said: "Keep a diary, and someday it'll keep you."
Truer words were never spoken. Successful writers journal. Your journal clarifies your thinking. It's a repository for ideas you'll write tomorrow and next year.
Journaling is also valuable as a writing practice: it keeps your mind focused on your writing.
Looking for ideas?
Keep a journal. Seriously.
I adore Day One (Mac.) There's probably a Windows alternative.