Capture it! Definitely works, and once you get it, you've got it. Use a clicker, or a verbal marker I suppose but a clicker would probably work more quickly, sit down with something to read, disguised treats in a bowl that she can't get to, clicker in your hand idle, moderately tired puppy on a leash. Ignore puppy. Wait as long as it takes. When she lays down, click out of nowhere and treat a bunch of times while she's still laying down. Then toss one, or if she gets up, then go back to your reading and puppy ignoring. Continue for about ten minutes and stop, even if she doesn't seem to be understanding, just be patient and try another session later. Keep doing sessions like that until she's offering the downs immediately after you click and treat the ones before. I did this with my puppy-- actually, I haven't used a single lure with her, and she was a very quick study.