To do this trick simply buy anything from the Bonus section until you dont have enough points to buy what you want. Leave the bonus section, and then load any of your gamesaves.
Once loaded and BEFORE the game autosaves bring up your PDA. You should notice that the amount of points being displayed is what you had before you purchased anything.
Exit your PDA and then get the game to autosave, and then head for the Bonus section once again, and you will have whatever amount of points you had before you purchased anything, meaning you haven't lost any points.