Of course high sticking is the best way to maintain a drag-free float, but you can achieve it through careful mending. When you cast upstream make sure you do a hard mend upstream and get your flyline directly upstream of your float, running with the current. Then use a float with a stick and watch the position of the stick. If the stick is pointing upstream, you probably have a pretty good drift. If you see it turn, mend your line or drag your line back upstream until it points back upstream. Fish will often strike when you drag it back up, so be ready for a strike.