Priorities are important!
Todo.txt uses capital letters to denote priorities. That means there are 27 priority values (A-Z and no priority), which is far more than you probably need.
This is how I recommend to assign priority values:
||The current task you are working on|
||Your next action|
||Tasks you plan to do today|
||Tasks you plan to do this week|
||Tasks you plan to do next week|
|no priority||everything else|
Of course, you can come up with your own system of assigning priorities.
I also recommend changing task priorities very frequently. I always want the tasks I plan to working on next to be sorted, by priority, to the top of my task list. To make sure that always is the case, I increase or decrease the priority of various tasks each time I review my task list.