Manually adding decorations to a house is a real PITA, but it can be done.
The only thing I've found that works is to drop the item once, pick it back up (take it into your inventory again), then drop it again, and then grab it and put it where you want it to go. When you've got things where you want them, make a save inside the house before leaving. Then knock on wood, cross your fingers and toes, pray to the deity of your choice if you have one, and hope for the best.
Typically though I make a save after placing or moving one or two items, or after finishing up one room. It's tedious but not so much so as coming home and finding your stuff all in disarray again. Also I've had a tendency in the past to accidentally hit the Shouts/Powers key with Unrelenting Force equipped. If I do, at least I can reload a save from inside the house in which I do not have to go back and redo every little thing after blowing the entire contents of a room up against the far wall.

For some reason those dragon claws are a real pain, I know I had a couple with my last PC that I could NOT make stay in one place no matter what I tried.
And just last night I bought Hjerim which is modded to have a library with display cases that you can put anything in and I thought YAY finally a nice place to keep dragon claws. Only have one so far so I dropped it and picked it up and dropped it and then put it in the case. Got it positioned all nice at one end with room for maybe 2-3 more claws eventually and turned to leave, only to find that my follower Stenvar - who was jus' chillin' in my crib - had wandered into the library to watch. As I passed I heard him say "I think you dropped this. I'm pretty sure you want it back" and then I hear the dreaded whooshclink of the claw being added BACK INTO MY INVENTORY.
*sigh*
So I had to do it all over again.
If anyone has a foolproof way of manually placing items and making them STAY there until they are manually moved again, please share. I do better with the drop/take/drop/place/save routine, but it's still not 100% guaranteed.
Edit @ klesh: had not heard about dropping items then leaving the house and coming back and placing them. I will have to give that a try, there are still a couple of stubborn items in Hjerim that will simply NOT stay put.
Edit redux: alas I'm wondering if this would cause a lot of hassle, I have a housecarl and a follower and I've had to reload previous saves twice in the process of redecorating because I would drop something and the two of them would then fight over it even after I picked it up again. Once it was a wooden plate, a WOODEN PLATE that I can't even sell to a used wares dealer but those two tried to kill each other over it. I'm just wondering what would happen if I dropped a whole bunch of stuff and left, I mean if my follower was following and left with me maybe it would be okay. Eh, I can always make him wait outside for the duration.