I have read plenty of unfamiliar code that was written by someone whose then-present motivations I simply did not understand, except for the comments that he left behind ... and that person was me. If you set-aside a program that you wrote, even for a very short time, then you will frankly be amazed at how little you actually remember of what you did – and why. The programmer’s intentions will now be an utter mystery. Comments and per-sub documentation is therefore quite essential.
I also believe that you should maintain your code in a change-control system and keep an issue-tracker ... even though you imagine that no one other than yourself will ever touch this code and that you will remember everything. You will not.
“Code review” awareness is also crucial – even if you are reviewing yourself. Would someone who had never encountered this piece of code before, manage to understand it ... accurately and quickly? Is the logic “stupid simple?” Yes, this is important, because soon enough that person will come along. And that person could very well be you.