Do they use an elastic strap on the inside of their pants, but if they do that wouldn't the very bottom of the pant leg kind of flop around a little bit??

There are a few different ways for players to get this look. Gary Sheffield gets his pants extra long and cuts the elastic a little at the bottom. And to keep the pants from riding up certain players use double sided tape. Another way is just to cut the elastic off completely and use the double sided tape to tape the bottom of the pants to the shoe. Most players just get extra long pants and pull the pant all the way to the heal.

They just get them made long, with no elastic in the bottom. Look at Andruw Jones for the braves. His are about 6 inches too long. I'm surprised he hasn't tripped over them. Maybe those long pants are why he can't hit.

The uniforms are tailored believe it or not.

If you saw the Bronx in Burning, Reggie Jackson made the equipment manager hem his pants during an interview he was having.

You can also by pants already made this way. They have the elastic strap to go around the bottom of your foot before you put on your cleat. If I'm not mistaken they are called the Clemson Cut pants and you can get them from a company called Victory.