Depends on what you need to spy about. Can you be more specific? The show "Cheaters" might be a good start, perhaps they have a website, there is software you can purchase to track activity on computers, theres website you can pay people to befriend someone and at the same time spy for you.
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You crouch a lot, where socks if you have wood floors so they don't hear you step (you should slide), make the least amount of movement as possible because your joints will pop. no sudden movements, it draws attention, and stay in shadows. if you are trying to spy on them when they are in a bedroom, previously leave something in there on purpose so you can go in than leave the door cracked on your way out. stay behind furniture and never go out in the open even if you don't see anyone. Stay against walls if you sit up (best not to stand), but if possible crawl on the floor. there is always the chance they will still see you, but this should help. good luck!!