how do you properly adjust trim once the boat is up out of the water? i see a lot of guys have a huge rooster tail coming out the back. is there any benefit to this or does it just look cool?

After you are on plane tilting the trim up pushes the stern down and in turn the bow rises.   This doesn't sound right but the net effect is less hull touching the water.  This in turn means less drag from water friction and more speed.  Once tilted too far up the prop starts to come out of the water and looses traction.  There is a sweet spot.

This doesn't take into account proposing, stability, or control though.  If any of these are outa whack put the trim down a little more.

The general rule of thumb is, on a properly setup boat,  the rooster tail shouldn't exceed the height of the top of the motor.  

Just trim it up until you like the ride.  It will vary depending on speed, wind and boat traffic.

Just trim it up until you like the ride. It will vary depending on speed, wind and boat traffic.

Good point.  Calm water allows higher trim adjustments.  Choppy water needs lower adjustments.  You should be able to feel these things while driving.