Theres not much of a system per say. I've learned with productivity systems, it's a minimum amount necessary, least amount of work type of thing. I've tried GTD, Things, iGTD, Outlook, several other todo apps and so on, but they are all too much work.
Now I just have the calendar app in my iPhone to remind me about appointments/due dates, and even then I don't bother putting most events in there (too much work).
The clock app for my alarm clock and parking timer and the notes app to keep free form lists of various things i need to do or get.
I find using notes is a lot faster than some list app, since each item i need to get is just a simple newline, vs a bunch of interface i need to navigate to add yet another item to my grocery shopping list. It's also with me at all times, and I can make all sorts of lists such as 'movies to watch', 'stuff i would like one day', 'stuff to get/to do', 'body fat measurements', 'corporation goals', and so on.
I think a good benchmark of a productivity/ accountability app is if it would be faster/simpler than using notes on an iPhone, if not, then it's not good enough yet. For example, WeightBot for iPhone (which i use every day) is nicer and faster than using the notes app for entering my weight, adds useful things such as graphs of my weight over time, has a goal chart, and has online backup/syncing for the inevitability of me reflashing my iPhone or something similar. I wouldn't use a web only app with my iPhone, since it's slower than notes usually, but an iPhone app with a web app compliment would be a big bonus. Some sort of passive tracking system like WakeMate or FitBit is also good for productivity tracking, a big part of personal productivity.
When I really need to plan something out (rarely) I get out a piece of paper and make a list/outline/milestones list. The rest I keep in my head.
I think things like GTD and such work really well for the overworked executive type they were designed for, while one for a student who doesn't have too much on his plate doesn't need much, or the other for the employee who just goes to work, and work is left needs only a scrap of paper for groceries. The self propelled entrepreneur needs another system.