Having been in this trap before, I've always put the rego dates, with three week preceding reminders, in my diary. Now a days it's in the I (everything) phone, because I've learnt:
The four greatest lies ever told are; 1. Your cheque is in the mail.
2. I'll still respect you in the morning.
3. I'm from the government, and I'm here to help you.
4. Trust me, I'm a lawyer.
Here endeth the lesson.