Developing calluses on the tips of your fret hand's fingers is necessary for playing the guitar. Playing the guitar as often as you can is the best way to build calluses, but there are ways to speed up the process.
Swab rubbing alcohol onto the tips of the fingers on your fret hand two or three times a day. This will remove excess moisture and help develop and maintain calluses.
Use water sparingly when washing your hands. Water can destroy calluses and force you to begin again. Keep your fingertips dry as much as possible.
Play the guitar at least 20 to 40 minutes a day. Use all your fingers as much as possible.
Use a product like Rock-Tips, which creates a tough protective membrane on your fingertips. It is made to both protect your fingertips and build calluses faster.
Rub your fingers on rough surfaces as often as you can, or carry something like a rough stone with you, so you can use it throughout the day to toughen your fingertips.