I line up chairs along the wall so that I can step from chair to chair to chair and put up the whole roll at once. If I don't, then it tends to come down when I re-position.
Also, start in the least obvious place. That is also where you will end (duh!) and your matching will be less obvious. If your border has a pattern, you will most likely end up with an unmatched pattern. When that happens, I cut out shapes from left-over border (the flowers or whatever is on the border), moisten them, and place them strategically over the seam. Most people will not notice a few extra flowers (or frogs, as it was for me last month) but they will notice a border that is not matched.
Good luck!