Like Atrocities said, what I do is build one sensor sat, in orbit around a well defended planet. If the sat isn't safe there, well, you've got bigger problems.
In enemy territory, I'll place the sat inside a storm. The standard game usually creates one per system. You get a nice system monitor with a fairly long lifespan.
In a nebula, its nice to build a base. Usually at the midpoint of the path between 2 warp points. Its nice if its a shield nullifying nebula, and the base is crammed full of armored fighters.
'Course none of this will fool anyone on Play-By-Web. You just simply expect these sorts of tricks to be in play. In fact, I've seen people just drop an uncloaked sat in enemy territory. You've got eyes for a couple of turns, that's all.