Without a good grip on your fly-fishing rod, you cannot reach your true potential on the water. A lot of people tend to grip the rod too lightly, which hinders their ability to cast the reel. You should aim for a relaxed give that gives you the most control over casting the rod.
You can hold it with your thumb on top, create a V-grip by turning your thumb, or rest your index finger on top. Try to avoid holding the rod flat in your hand and keep your palm relaxed.
Check out this video for more tips on holding the rod before your next fly-fishing trip to Montana. When you can focus on the fundamentals, it helps you perfect your form.