The best way is to reach over your head with your opposite hand, grab the top of your ear, and pull up and back while inserting the earplug. If it's the foam kind, hold it until it expands. This provides the largest volume of space in your ear canal, allowing the deepest fit/widest expansion. To really go nuts, open your mouth/lower your jaw at the same time.

Once you understand the principle, you can do it one handed - I hold the earplug with my thumb and forefinger, using my middle finger to push my ear up and away using one of the inner pinnae.

See http://ehs.sc.edu/modules/Noise/plugs.htm for an picture.