One way of telling your parents would be to download some of the most coherent videos on Atheism from the internet (something like 'The God who wasnt' there", for example) and burn them onto a DVD (or Blu-Ray if you want to go hi-def) capable of being played in a common or garden DVD (or Blu-Ray) player. You could have fun with the menus and such, or go all out and put a personal message at the beginning of the video. Nero and its ilk are good for this. This would then be presented to your parents in the form of a present. If you use the correct type of disc (the ones with a white top I think), you would be able to print on the disc as well to make it even more professional-looking. If you're feeling particularly ballsy, you could watch it with them and answer any questions they may have.
Another way would be to ease them into it with a slow realisation that, for you, there is no personal god. Start off with a small doubt being voiced. At this point, you will most likely be mildly assaulted by reasoning from your parents on the existence of god, depending on how strong their belief is. You would then slowly move towards Agnosticism using logic. This could then be enhanced by you visibly researching the issue and slowly coming to the realisation that the facts don't support the hypothesis and that you don't believe. It's called incrementalism and has been used by governments around the world throughout time to slowly allow their populaces to become used to an idea or principle. Maybe not the quickest way, but it has the potential to bring other people with you not by force, but by reason. Reason is a powerful tool if used correctly and should always be preferred over force in any given situation. This way will most likely be very difficult and allow far more time for others to attempt to convince you otherwise, but stay the course and I think it may well pay off for you.