sneak w/ invisibility potions. Also, build your range up a bit... Always helps.
There are some amazing "sneak perks" that will help you out. utilize them, also- the shout "throw voice" is pretty handy.
This is basically my formula: Sneak mode, keep moderate distance and try to stay behind target. If target faces you, watch his movement cycle and note any particular moments when his back is turned to you. Once his back is turned make your way behind him, there's a "roll perk" that will shorten the distance quickly. Then with a dagger, attack unsuspecting enemy- there's an assassin perk that allows you to do 15x the damage with a dagger in sneak.
If the enemy is alerted to your position, toss back an invisibility potion, sneak to another area, give yourself some distance if you have to (hurry if it's a 'brief potion'), and make a plan of attack based off their movements. Always wait it out if they're 'alert.'
If the enemy is alert but doesn't quite know where you're at, and they are surely going to make their way to your position- use that throw voice shout (point it in a direction furthest away from you) and when they seem distracted, pull back, or if you see any opening for an attack- use it.
Again- utilize your ranged weapons as well. Staying in sneak mode, there's a perk that will do 3x the damage with a bow (or crossbow). Distance is your ally. It gives you time to reload and make the kill, also- it makes it harder for the enemy to find you.