Two main things. One is you need to know how the top goes on correctly.
Two are things you can add to take up any slack with a loose/stretched out top.
tailgate = adjust, then replace weatherstripping
top = pull it as tight as you can get it with the velcro straps in the back. I used to put the foil insulation above my head between the factory bars/cage and the top. you know, the bobble wrap two sided foil stuff. It helps in the winter and summer. I also used to have it under the carpet back when I had carpet. Put single sided black sticky foam insulation between each snap along the back and sides. Again, helps also keep out the cold. Make sure you have pulled down the sides that slide down the grove as far as you can get it... it matters. That is what makes it tight against the targa bar.
The little velcro things that go around the bow should not be missed. They hold the top to the bow so it doesn't flap.
the velcro over each door should be pulled as tight as you can get it. if the velcro doesn't light up just right I've seen people put foam pipe insulation around the parts just above your head.
The reason why it's quieter with the back open is the air has a place to go out. (air pressure & just plain ol wind) Notice how it changes with the windows up?
Try the windows up and the back open. Think about it like this, with the windows open you are trying to fill up a big balloon. With the back open the air is going out the back, causing the sides to wave in the breeze. I've had mine from new in '87 and have a lot of miles on it. I think I've every stupid trick I heard. Some worked, some not. Mine hasn't been a daily driver for quite some time.
I'm sure there are some other things I've forgot and others tricks others on this list they can add...
But then, maybe you just need a louder radio.