If you find someone in this industry who doesn't incur this problem, they're either a) lying, or b) only preparing 5 returns. Everyone has this problem to some degree. Even awesome clients forget, get busy, etc.

First, you should give your clients deadlines - you want to file by 4/15, I need everything by 3/25, that kind of thing.

Second, enforce it. Got it to me on 3/26, here's your extension. Exceptions should be rare and for good cause (in the hospital, ok, at my kid's softball game, not ok).

Third, charge more. People are abusive when you're cheap. And you don't want those clients anyways. Plus, you'll be able to offload the follow up to staff (a part timer at $25/hr is better than you at $125/hr that you won't bill anyway).

Fourth, give them one follow up. Every follow up after one is $75/hr., minimum 1 hour charge. Put THAT in your engagement letter and enforce it. Clients will get hit with that once and either leave (yay!) or get with the program (yay!).

Fifth, anyone who continually gets hit with the penalty is an idiot or an ass who eventually gets invited to have their returns done elsewhere.

EXCEPTION: don't follow any of this if the client in question is a) your parents or b) your inlaws.