Airlock activity is a poor indicator of fermentation activity near the end, so it may still be fermenting fine. The only way to really tell is to wait 3 days or so and check the specific gravity again to see if it's is dropping. You may be just fine, so if you aren't in a huge hurry I'd wait it out. If you're in a hurry and kegging (bottling risks bottle bombs, been there done that), you can go ahead. The beer may just be a bit sweeter than you were targeting. (You can also keg a gallon or two and keep fermenting the rest.)
In addition to pitching adequate healthy yeast (use Mr. Malty's pitching calculator) as you've noted, I've had good luck with increasing the temperature to 70-72F for 3-5 days once the airlock slows to a dribble. With the bulk of fermentation done you don't get off flavors, and the warmer temps help the yeast finish up. You can also roust the yeast by swirling the fermenter once a day as well. If it truly sticks and you really want it to finish, you may have to rack it to a yeast cake from another batch (been there, done that too).
John (Tristinn's Dad)