There is a trick to getting the poles to stay up nice and tightly and not slide down, you need to tie them something like this…
You need to wrap the long tie, around the outside of the poles once to hold them in place, then wrap the tie up and around the poles as many different ways as possible. This way the poles are held tightly, and slightly outwards, keeping the sides of the teepee taught at the top and stopping the tie from sliding downwards.
You may need to re-tie the poles like this from time to time if you are keeping you teepee up for long periods of time as the fabric tie can stretch and loosen, especially when it is new.
You can see even more pics of various ways to wrap the tie up, under and around the poles over on Flickr here. Happy camping!