I've been on my own now for 8 years (as of yesterday!), and have changed a few things to avoid the last minute rush:

- my client base has changed from sole traders and partnerships to limited companies, so the personal return aspect is easy (dividend and salary for most of them). This has the added benefit of smoothing out the workload, as December is by far our most popular year end
- we've put a reminder system in place so that clients get regular reminders throughout the year
- when I get to early December, I list the outstanding returns to focus. In early January, I split between those that I think will get submitted and those that won't. Cavalier I know, but I don't want those who care to suffer at the expense of those who don't. This year, all the ones I expected to get completed were completed. There are currently about 7 returns outstanding for clients who don't care, don't respond or don't understand! I've kicked them enough, you can take a horse to water etc..
- we now get the messier accounts (the ones you don't look forward to doing that take time and thinking about!) done and out of the way early in the year

I've also learnt that, at the end of the day, the penalty is £100, and some people will pay it. Of course, the penalty system has changed this year, but unfortunately some people will learn the hard way.