OK, so I have read from many people that wrapping your baby on your back was not only possible, but quite easy. I have not found that to be the case. However, I have successfully put my baby on my back several times. I don't feel very comfortable though. Here is a picture (it's crummy because I took it in the mirror) of Daniel on my back.
Tube sling: Great for brand new baby. However, this type of sling only allows for putting the weight on one shoulder, so once the baby starts to weigh much, the baby gets heavy quickly.
Mei-Tai: This sling is wonderful, but wearing the baby on the front, facing out is more difficult. You can do it, but just not for long periods of time. However, this one is awesome if you want to put the baby on your back. It distributes the weight on to both shoulders, your back, and your waist. It's like giving the baby a piggy back ride but without you having to use your arms. I can carry my baby quite comfortably this way for several hours.
Wrap: I have tried three different wraps. I have tried a 5 yard knit wrap, a 5 1/2 yard knit wrap, and a 5 1/2 yard woven wrap. The bigger the child gets, the easier it is to use a woven wrap. The woven wrap seems to me to hold more if the weight of the child. The knit wraps are easier if you want to be able to take the child in and out. The 5 1/2 yard wrap is more versatile, but in the end, if you don't mind tying in the back, the 5 yard wrap is easier to deal with. The tails don't seem to go on the ground as much. This wrap is wonderful for any of the front hold carries, but I have not yet found a comfortable way to put the baby on my back for a long time. I think it may just be that I need to do some more adjusting with certian parts of it, but it is still time consuming, and I would rather not spend the time when the Mei tai works quickly.