Ok I had this idea, and I am not really sure if it would work but I think it might. Most people write in a dream journal to help them remeber a dream. So what if you picked something simple you want as a dream sign, then every time you write in your dream journal you say you saw the sign. Maybe you can even picture it in your head when you write it down.

If you did it with every single dream, and you did it long enough, you might just start seeing the sign for real. If it worked and you kept doing it you might start seeing it in every dream you have. Then you would have a sign in every dream, that you never see in real life that is easy to notice. You might just be able to become lucid all the time.