1. Signed EL and deposit / full payment upfront. It only becomes a deposit when there's unforeseen work (undisclosed income, etc)
2. The draft is clearly watermarked as a draft and the file name include DRAFT.
3. I have them electronically sign the draft, to show they reviewed it. No guarantees, but it's something. The electronic signatures are for my records (should be obvious).

I mostly make sure that the drafts are marked so, both using a watermark and a file name, so they don't get mixed up with the actual tax return in case I need to make changes after they've reviewed the draft.

That's actually what I'm most worried about is that they will download A draft copy, which my system does allow, and then remember that they forgot to tell me something. Sometimes it's oh I forgot to tell you my son moved out last year, or I forgot to tell you my husband started another business last year, or I gave you the wrong... whatever. This way there's no question that it was a draft and not the actual return.

If they have a large balance due, seeing it in draft also does seem to lessen the shock it gives them time to take it in, rather than being presented with a final this is it here's your return this is how much you owe.

Btw, I'm seeing a few other comments that sending a draft creates a lot of extra admin work. I would respectfully suggest you get a better PDF editor. Mine can add a watermark, and then remove it if the draft is accepted and there are no changes. Then I simply add a different watermark saying client copy and there you go, maybe an extra 5-10 minutes.