Here is a tip or two:
If you have Emerald, go to the Fossil Maniac's house and catch a Ditto with the Nature that you want. Next, have the Ditto hold the Everstone (found in Granite Cave) and breed it with your female Lileep. Save your game when the Daycare Dude is in front with an egg BEFORE you take it. After saving, take the egg, make sure a Slugma (or other Poke that has Flame Body/Magma Armor Ability) is in your party, and walk around until the egg hatches. You have a 50% chance that the egg that hatches will have the Nature of the Ditto. When it hatches, DO NOT SAVE. Check the gender of the baby. If it is a female, check the Nature. If the Nature is correct, you now have the female to use to breed with your male Corsola. If the gender/Nature is not right, turn off the game, and DO NOT take the egg from the Dude. Let him keep it, and you walk around until he has another one. Save BEFORE you take the new egg and repeat the process. This will not only cut down on the time it takes you to breed correctly, but also the amount of eggs you will get from it.