[A counter against flying types? Someone's already doing her research! Good. He likes to see that.]

Mainly? Keep it warm, and keep it near other Pokémon. I have many working theories about the relationship between Pokémon growth and energy storage, but to put it in plain terms, the egg will need to absorb some measure of energy in order for the Pokémon within to develop to a point where it can viably hatch. Simply carrying it with you as you travel is usually enough.