If the river is large enough, fish down and across with a slack-line dry-fly cast to the trout. This means that the fly floating downstream will drift into the fish's view before the leader. It's imperative there be no drag on the fly. To avoid it, feed slack line into the downstream drift a little at a time by shaking your rod tip gently. If the fish doesn't take, allow the current to swing the line, leader, and fly well away from it before you pick up for another cast. On smaller streams, where there isn't enough room for down-and-across fishing, you'll have to work upstream. Remember to keep a low profile-on your knees, if necessary-and to keep your false casting off to the side and away from the trout's view.