Firstly it helps if your a ninja or have ninjutsu/genjutsu abilities. Taijutsu is non applicable here.

• Being able to fold into the wind, so if the person being spyed upon senses you.
• I like to use my preripheral vision this way I can spy on most people and get away with it right in there face!
• Knowing where they are at all times helps i.e. take mental notes of where you see them for future reference. If possible get a copy of there timetable.
• There is no real safe distance to spy but on the move it can be between 2 - 10 metres away from the target. Stationary about 10 metres+.
• Be aware of look alikes that aren't the actual target. You don't want to be lead of only to find out its some ugly person. Like the ninjutsu power of cloning yourself, you must be able to see through these immatations.
• Keep your head on a swivel