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Not really a fast method but constant.
Set a waypoint to Trevors airfield if you don't own a helicopter or call one in from merryweather and kill the man.
Then simply rubber band R2 trigger down or wedge a bit of card in the R2 tigger so you gain altitude and go higher.
Every 10mins will add 1% to your skill.
I haven't tested this properly but you can get XP/RP from this though, get a few of your friends in the helicopter too, you will all get xp for flying with friends after some time
New Faster Method:
Go to a airport, using a stunt plane or a small agile plane, take off and land loads of times literally as soon as all your wheels are off the ground land again do this for the whole run way then turn around and do it back up the other way.