I don't have a dog, so all my flushes are very close. I use 6s in the first barrel, followed by 4's in the second - l've also done the same with 7.5's and 6's. The birds I come across are either stocked pheasant or native grouse, so they die a little easier than the wild pheasant most of you are lucky enough to be able to hunt (before anyone yells about me using 7.5s - they've given me clean kills every time I use them).
I also do something similar with my repeaters. I put a 4 in the mag first, then 2 6's (or a 6 and two 7.5's, depending). This way, by the time I get to the third shot, I'm now using a larger shot for what should be a longer range.
I never load more than 3. If I can't hit with 3, I'm not going to get better with my 4th or 5th shot. Also, anyone hearing someone stop with three should figure I've finally connected, and won't come running over to find the bird I missed, giving me another opportunity for the getaway.