I have one big to-do list for a project. The list is organised by topics. If a new idea comes up in the context of something else, it is put on the list instead

I have one big to-do list for a project. The list is organised by topics. If a new idea comes up in the context of something else, it is put on the list instead of being implemented immediately. (Exceptions are made for important quick fixes or very low-hanging fruit.) From the big to-do list a selection is made for a short work-in-progress list, largely in consultation with the project client. I only work on the items on the work-in-progress list. The work-in-progress list itself is roughly ordered by importance, but may be shuffelt around a bit according to new circumstances, insights or favourable opportunities (such as implementing one thing on the side when I am actually implementing something else). If the work-in-progress list becomes too long because the client or myself think some new ideas are really urgent or important, some other items from the work-in-progress list have to be moved back to the big to-do list. Following the Pareto principle, this may also happen for items which have not been completely but sufficiently implemented. From time to time the work-in-progress list will be completely revised. My guiding adage is: You can only work on a single thing at a time; always pick the most important.