How do you make your hair wavy forever?

If you have thick hair then cutting into layers will help you get some waves in your hair, if you have thin hair, then you are probably out of luck, unless you curl your hair or put it in braids every night. you probably want to talk to your barber.

EDIT: If your hair isn't already naturally wavy then your hair won't be wavy forever and the density of your hair doesn't really make a difference to whether your hair is straight or not (as above user suggested). Temporary waves can be achieved by either; braiding the hair and leaving in for long periods of time (overnight etc), curling with straighteners/tongs and brushing the curls out and even setting it in heated rollers or setting rollers and brushing the curls out. For permanent waves I would suggest getting a perm using the largest rollers possible which will last until it grows out.