I got an egg in a link trade what is the best way to get the egg to hatch?
If you have a bike, ride it on the long path before Celestia City. Up and down till it hatches.
If you have a Pokemon with Flare Body, bring it along for the ride, too.
If you got the train station in which ever city that is there is a circular section and if you have a 3ds you can put a coin in the circle pad but if you have a normal ds just put some pressure on the d-pad that isn't your hand and this works because it is a circle it will never stop and you will be able to hatch your eggs and if you have a Pokemon in your party with either flame body or magma armour it halves the amounts of steps required.
Flame body: http://pokemondb.net/ability/flame-body
Magma Armour: http://pokemondb.net/ability/magma-armor
Hope this helps