Oh, you must keep a sketchbook! An architect explains why all creative people need a sketchbook or a special notebook for ideas
https://www.youtube.com/watch?v=-ddxBY9pUfA

Even if it's not in a book, staple some loose papers together, or connect it with a bit of a string. Come browse other artists' sketchbooks here:
https://www.youtube.com/watch?v=oIWO_LBwjqI&list=PLSyQ0NEnXiTQnuAKfU73WGPqAU8QxVd4K