Keep a notebook with out at all times to jot down those random, incomplete ideas you have over the course of a day. Then when you sit down to work you can just

Keep a notebook with out at all times to jot down those random, incomplete ideas you have over the course of a day. Then when you sit down to work you can just go through your list and throw away the mostly bad ones, but hopefully you'll have a good one in there worth expanding on. Either way at least it gives you a starting point to actually start working.