If you have missions in Las Venturas and you are in Los Santos or anywhere, it will take ages to get to do those missions. Don't bother getting your own plane or driving all the way. Go to an airport and go to the main entrance. Heres where they are:

Los Santos: In the airport base you will see an inner rectangle. Go to the bottom left of that and search.

San Fierro: Just go to the right hand side of the circle on the map and go on the upper part of doors.

Las Venturas: Im not sure... look around the gates of the entrance to the airport runway.

Once you have found the entrance walk into the yellow marker. A screen will come on showing the ticket you can buy. Scroll through the tickets to the airport you want to get to. Then Click X and you'll be off. In the middle of the flight, If you want to, press triangle and you will have a parachute for fly about on for a bit of fun. Or press X to get to your destination way quicker. Don't press anything and you can watch your flight. It might get boring.