Sometimes enemies are at the "wrong position" for a perfect back-stab assassination. How can I get enemy attention in order to move him to a better position to perform a single hit kill with knifes?
WHAT WAS THAT?!
I find the easiest route is to step a little bit into a lit area - just enough to get the eye to start opening. This will cause enemies nearby to draw their weapons and begin to search for me. Then I retreat into a corner, nook, or on top of a nearby object and wait for them to path around into the darkness where their back is turned. Then, go in for the kill.
Bonus points: Lead them into a trapped area, and watch them repeatedly trigger the traps and kill themselves.
Firing an arrow into a wall has been suggested, and it's a decent strategy, although if you have the relevant perk from the Sneak tree, you can just shoot them directly - it counts as a sneak attack, and can 1-shot weaker enemies such as skeletons if your archery skill and damage are high enough. Hitting a stronger enemy from the shadows doesn't tend to fully expose you, and they'll path around as if they'd heard or caught a glimpse of you before giving up and turning their back.
Say hello to my little friend!
Conjuring a friendly beast also works well, assuming you've got silent casting from the Illusion tree (or you're far enough away that they won't hear you). Your conjured beast will engage them in combat, which moves them around the room and can do significant damage, while keeping the focus off of you, so you can sneak in for easy kills. This works especially well if you flank multiple enemies while they're distracted - you can take out archers and mages pretty quickly with this. I tend to prefer the Flame Atronach, which doesn't require high Conjuration skill, but does good ranged damage and explodes when killed.
Can't hit what you can't see...
Invisibility makes this trivial, since as long as you don't touch them you can move around in the light as much as you care to. In addition to potions and the spell, the abilities Shadow Warrior (Sneak 100 perk) and Shadowcloak of Nocturnal (reward for a Thieves' Guild quest) can grant you temporary invisibility without the need for a high Illusion skill or the proper consumable. These two abilities were compared here.
Shadow Warrior doesn't last very long, and it seems a bit weak at first. However, for the instant after you crouch it you're considered invisible and undetectable, which means that you can get credit for backstabs even if you're in active combat with someone, if you move fast enough.
Couple things I have done to make enemies change their positioning.
I will shoot an arrow to a corner in the room or a good shadowy position in a cave to give myself some time to get behind them and do a quick backstab, this has the slight drawback of not holding their attention for very long and they will probably find you if you dont move.
There is a shout called Throw Voice that will do the same basic thing as shooting an arrow with less chance of being detected and holds their attention longer. Although the drawback is every enemy in the area will be drawn to it so expect lots of enemies to come clamoring towards your shadowy corner.
These are the only two ways i have found in my days in the theives guild and the dark brotherhood.