To Do Lists
I've been coding solo for quite a while now, so I've not been using a issues tracker database, and instead I've been using a simple text file.
But even if I were to use an issues tracker, I would still use a to-do text file.
Each time I implement a new feature, or fix a bug, I write the task at the top of the file. If it is complex, I break it down into smaller parts, and write a line for each sub-task.
Mid way through coding, it is common to realise that there's an unexpected side task. I write it in the to-do list, and now my mind can happily forget about it, and get back to the work at hand.
I'm surprised how useful this is.
There is a down-side. My to-do lists almost always grow, and rarely shrink! Today, one of my project's to-do lists became empty, and that was quite shocking. It's never happened before!