Why did the wampa put Luke Skywalker upside down in the ice cave on Hoth?
After I kill a chicken, in preparation for plucking and eating it, I will string it up upside down in order to get all the blood out. Blood in a meal just isn't tasty.
Assuming the Wampa has intelligence but no knife, this would be good prep for a meal. And, if he has no knife, it's a good bet he doesn't have rope either. So, he makes do with body fluids and materials in the cave.
If you are going to immobilize your prey, prepping in this fashion makes sense.
The problem, of course, is that as long as Luke's heart is still pumping it is ineffective. I am assuming the Wampa is therefore mimicking the ceremony (much as a dog might) without understanding the purpose himself. Alternatively, as he had just eaten the Tauntaun, he may have gotten things ready, so that when it came time to eat Luke, he'd just need to slit Luke's throat to drain the blood. By not killing him, Luke's meat would still be fresh - although Hoth's environment is such that spoilage wouldn't really have been an issue.
The Wampa was storing Luke for his next meal. That is why in the extended version of Empire with the added scenes, you'll notice that the ice creature was just finishing up another meal. You could see the bones on the ground. He was just about to start going for his next meal (that being Luke) when he awoke. Then with Obi-wan's help in spirit form (as shown in the novelization), he was able to use the force to pull his lightsaber into his hands, cut his bonds and escape, thus saving his own life.
Regarding Obi-Wan helping Luke, in the original Star Wars (A new hope), remember that Obi-Wan was there talking to Luke and helped guide him as he blew up the first death star. He was also protecting Luke as Vader attempted to destroy him in his TIE fighter. That is why Vader uttered the phrase "The force is strong with this one" because Vader was attempting to use a dark side technique to confuse his opponents. This is one of the reasons why he was such a good fighter pilot. Notice that with Red Leader, Biggs and others. they suddenly became confused, looking left and right. This is Vader using that power to scramble their brains and confuse them, turning them into sitting ducks.
He was also trying to do this with Luke, but it was not working due to Lukes own natural force power and Obi-Wans help, again causing Vader to say that the force is strong with this one. This is shown in the Extended Universe book "Heir to the Empire" where Luke was used this same technique to confuse TIE pilots and felt it was very close to the dark side, so he didn't use it anymore.
Also in The Empire Strikes back when Luke refused to stay on Dagobah and Obi-Wan warned him 'If you choose to face Vader you will do it alone, I can not interfere" further supporting the idea that Ben was protecting Luke and adding him power to Luke at key points too.
If the wampa wanted to drain the blood, presumably he would also want to cook the meat. raw meat carnivores don't typically drain the blood of their prey. I would think he was placed there so that he was just out of the way. The wampa got home, put Luke's feet up to the roof, hocked-a-loogie on them, and just held him there until it froze (and mysteriously didnt freeze Luke solid), and ate then ate some Tauntaun.
The very excellent (and Disney Canon) "Star Wars: Absolutely Everything You Need to Know" offers this information about the feeding habits of Wampas
Death by Wampa
Ambushes and stuns its unsuspecting quarry
Lugs its prey's limp body back to its cave
Suspends its victim upside down to gnaw at later
Devours the prey at its leisure - frozen or fresh