While some lawyers may suggest you keep a pain diary on a day-to-day basis to record the problems you are having after an accident, the decision to have such a diary is really a personal decision. The difficulty with having such a diary is it focuses you on your problems, which is not a good thing psychologically. The advantage of having a diary is that you can refresh your recollection of the events after an accident when you are asked about them years later.
In terms of the diary, what you should do, as a minimum, is keep at least a calendar to show when you attended various appointments. The calendar should also note significant events that occur after the accident such as when your back problems stopped bothering you, when you returned to work, etc…