Instructions
Take the Pokemon you want to mate to the Pokemon Daycare Center on Route 34, to the south of Goldenrod City.
Place the Pokemon you want to mate in the Daycare. Using the same species of Pokemon but with different ID numbers in their profile gives the best chance of them producing an egg quickly. Talk to the man outside the Daycare center and if he says "The two seem to get along very well!" then your Pokemon have the best chance of producing an egg quickly..
Walk around for 256 steps and talk to the old man outside the Daycare again. If your Pokemon have mated he will give you an egg -- if they haven't, walk around for another 256 steps and check back again.
Place the egg in your traveling party of Pokemon along with a Pokemon with the Ability Flame Body or Magma Armor. Having this Pokemon in your party with the egg will make it hatch twice as fast. Common Pokemon with Flame Body or Magma Armor are Magmar, Slugma, and Magby.
Keep moving around -- every 255 steps, the egg will have a chance of hatching. Stay out of long grass and avoid other trainers so you don't waste time on battles. Use your bicycle and go up and down the cycle path, Route 17, to cover a lot of steps quickly.