It's certainly much more advantageous when you see the enemy first and not the other way around. Ring of Fog helps a great deal with that, but you still need to walk instead of running to stay hidden longer before enemy sees you. But what other tricks are there to it?
I've noticed how different armor pieces produce variable amount of noise while moving, but does it matter in terms of stealth?
Also, shooting arrows near enemies will distract most of them and they'll turn around to the source of sound (where the arrow or a spell hit) and they'll stand like that for several seconds, allowing to sneakily get behind them and score a backstab.