I have some couches, shelves, etc. that I need to move up and down 1 flight of stairs (the stairs also include a turn/corner). Are there any methods/tools to do this when 2 people aren't strong enough to carry the weight?
Use a winch or a come-along to pull the furniture up the stairs. You may need to anchor it to a wall, stud, door, or something else that will not move. You will also need some type of straps to wrap around the furniture. These can be purchased at any big box hardware store, auto parts store, or moving company. Lastly, you will need a method of making the stairs slick enough that furniture parts that stick out don't get caught. I have found that a piece of plywood laid on the steps usually works well for this. Placing a blanket between the plywood and the furniture to be moved cuts down on the friction between them.
If you have to turn a sharp corner, you can use a pulley to change the angle of the cable (providing that you can mount it safely) or a device (a block of wood) that the cable uses as an inverted pulley to change the angle of the pulled item.