One would be proper temperature. The proper temperature for eggs is 99-101 degrees. You will need to keep this temperature constant. Even if the temperature changes from 101degress to 115 the chick can die. Keeping a steady temperature is very important especially during the start of incubation.
Another important thing is humidity. To hatch chicks the humidity needs to remain constant. Humidity should be 60-65. (83-88 wet bulb humidity)
in a forced air incubator. If you are using a still-air incubator then humidity should be 60-65 (80-90) wet bulb). At hatching time the humidity should be 70-75 (92-97 wet bulb). High humidity causes chicks to hatch ahead of time. Low humidity causes chicks to hatch late.
If you do not wish to buy a wet bulb thermometer you can make one
http://www.tifton.uga.edu/tobacco/wetbul…
Also if you are good enough at candling you can determine how much humidity the eggs has
http://www.extension.umn.edu/distributio…
Another important factor is turning the eggs. The eggs will need turned 3 times dally (there isn’t a specific time when you have to do this during the day. Just try to even the times out to fit your schedule) Turning the eggs prevents the chick from getting stuck to the side of the shell. Not turning the egg will kill the chick or make it imposable for it to hatch out of the shell from that position.
Stop turning the eggs on day 18. This is when hatching time may start and the eggs should not be turned again. Chicks should hatch day 21 but they can take till day 24 to hatch. Anything longer may be unhealthy.
Other things to keep in mind-
Washing eggs- Do not wash the eggs. Washing eggs causes the egg sealant to come of thus letting bacterium into the egg. If you must wash the eggs do it with luke-warm water.
Refrigerating the eggs- The eggs must have never been refrigerated or kept in a place under 40 degrees. If they were then the membrane of the egg is dead.
Marking the eggs- It is best to mark your days and such on a peace of paper next to your incubator. If you must mark the eggs use only a standard lead pencil. Anything else can soak through the shell (no matter if the sealant is there) and kill the chick.
Eggs not to use- Do not hatch extremely large or small eggs. You want the chicks to be healthy. If you have a double yolk and decide to hatch it, it will be very likely that the chicks are not healthy.
Over 10 days- Eggs that are over 10 days old should not be incubated. If they are there will be an increase in lameness and illness.
Make sure that the parent stock is healthy- If the parent stock wasn’t given proper nutrition or has any illness it could be passed onto the chicks.
Seeing if the eggs are fertile- To see if you eggs are fertile you can candle the eggs.
http://shilala.homestead.com/candling.ht…
