If you want to roam the wasteland with a permanently-enabled invisibility cloak like some kind of Boston-based Predator, then there are certain conditions you need to meet. First, you need your stealth skill at Level 5, and a piece of armour with either the chameleon perk, or Power Armor with the Stealthboy mod on it. Wearing one of the aforementioned armours, crouch for a few seconds to enable the invisibility cloak, then rapidly stand and crouch, stand and crouch several times in a row. If you did it correctly and fast enough, you'll find that when you stand up and start moving around, you'll remain invisible! You don't even have to continue wearing the armour with the invisibility perks. To come out of invisibility mode, make sure you're wearing the armour with the invisibility perk, then crouch and stand up again.