Flagging is the best idea since sliced bread. I remember in the early days when we would use a black square of material attached to a round dowel. You could wave it to passing geese that were far away and get them to swing in on you.
Being friends with the Flagman Randy Bartz, I've seen the flag design change over the years. Randy seems to always come up with some new ideas, which we're happy to try out for him. Remember, flagging works great but it can really hurt you if it's not done properly. I've witnessed people sticking flags up high when geese are close and have them flare out on us.
The key is flagging just a little when the geese are close. Use little flips up and down and is all it will take to put them in your face.
Flagging becomes really deadly in the late season. The birds tend to be wary of motionless decoy spreads and bad calling. Use a little flip of the flag to catch their attention. Then flip it again and greet them with your call. A lot of times they'll call back at that point and you can start talking them in!
Here's a picture of the guys flagging Geese with 2 different types of flags.