Be quick! When trying to get some great informal and natural shots of adults or children, make sure the exposure is set correctly on your camera.
Do a test shot, and you could even pre-focus.
Then, holding the camera at your side so as not to attract attention, wait for the moment, and practice raising the camera and taking the picture straight away in one smooth movement before the person realises you are taking
This is how I take my informal wedding images.
If you have any questions about any aspects of photography or camera choice, please email firstname.lastname@example.org and he will try to answer your questions.