I’ve always thought that this would be a great idea:
The concept is that the user can set a certain percentage of what they spend (which could be fine-tuned to only certain types of spending, or within a certain spending range), which is automatically ‘saved’ by being ‘charged’ at the time of the transaction, and gets sent to a savings account - which, ideally, would be hidden in some way from the user and/or just more difficult to access.
For example, I set my ‘saving’ rate to be 5%, and thus every time I spend (say on transactions below £100), I automatically charge myself 5% more, which is sent to my hidden savings account.
After setting this up, I probably forget about it, and then (because the savings account is hidden), in time to come I suddenly remember I have been secretly (from myself!) saving money… So I figure out where that savings account is, and discover a nice cash hoard!
It’s all about saving a little a lot, and the ‘hidden’ element is really trying to take advantage of (or reduce) the psychological bias/tendency for us to factor in our savings/earnings into our monthly discretionary spending habits. (Resulting in the oft-felt sensation that even when we get a pay rise, we miraculously seem to have the same amount left in the bank each month!)