secret agents anti tracking technique?

Firstly, I don't think you're being followed by secret agents. Don't worry. MI5 have bigger targets to worry about.

Generally if you want someone to stop following you, then you need to find a way to break the line of sight. Once you've done this, you should change clothes. If you are wearing a jacket or coat, take it off and ditch it (or wrap it up tight and hold onto it or stick it in a bag). If you have long hair, try putting on a hat of some kind and placing your hair inside the hat. If you have glasses you can take these off. There are all sorts of things you can do but generally the goal is to hide, and then change your appearance so that you can stop hiding.

It's not as hard as it sounds, but if you were genuinely escaping MI5, the FBI, the CIA, the DEA or any other large government organisation your best bet would just be to flee for the country side. In a city you don't have much hope getting away from a multi-million pound organisation with access to CCTV, computer systems around the area, and of course the local police.

Hope this helps ;)

I guess this is a hypothetical question. However, I can assure you that if you are a 'target', you would be hard pressed to know you are being followed. If you are a serious target, several operatives would be detailed to observe you. Some would be in front of you, some behind you, some in cars and your changes of clothing would make little difference as the operatives are well used to this practice.

It depends on how they are following you...
If there is a foot/car tracker, then it will depend on how many there are, how much traffic there is to get lost in, etc. If there is a helicopter or drone, then you have to get under something to avoid being seen. If they have a radio tag, you need to lose the tag or jam the signal.