here are the things that you might want to consider:
1. Use a long zoom - a telephoto lens allows you to capture photos without going close to your subjects. Since the people are not aware that you are snapping those shots, they act naturally. My advice, use a 200mm telephoto lens or longer. In my case as a hobbyist with an entry level camera, I used 55-200mm 4-5.6G VR lens. If you can afford a 200mm lens of 2.8f , why not!!!!
2. Dont use flash---remember that you want to work as covert as possible. The flash will make you noticed so better work on the available light. If it is dark. Increase your ISO and set your aperture to the lowest possible value.
3. Shoot as many frames as possible-- the more photos you take, the more chances of getting a perfect shot. You have to remember that your subjects movement is not in your control. Some keep moving so better shoot as many frames as possible.
4. Your location or position is very important--- anticipate your subjects activity so that you can position yourself strategically and therefore secure good angles. If you are watching a basketball match for example and one team is obviously dominating the game, you might want to position yourself on the winning teams side.
5. Capture people doing things---and therefore doing what they love the most. This means a lot of movements so if lighting is good, better use fast shutter speed so as to freeze the motion. If it is dark, follow the tips in # 2.
6. Photograph people with people – When you capture people in pairs or groups, you’ll capture a wide variety of facial expressions, emotions and responses.