Just like with most airports around the world, taking the taxi is the fastest, yet most expensive way to get to the city. A one-way trip would set you back around RM120 or so, depending on the exact location that you are going to. When you arrive, you will find several counters selling taxi coupons at the arrival hall. You should purchase your taxi coupons from these counters, as they are government regulated with fixed prices. You can choose to either make the trip using budget taxis, vans, or luxury limos. The journey to Kuala Lumpur city center takes approximately one hour.
The KLIA airports is served by an express train, the KLIA express that can get you to KL Sentral Station, near the heart of the city in just 28 minutes. It is by far the fastest way to get to the city. Tickets cost around RM35 per person per way. Although it might be the fastest, it is not necessarily the most convenient. Depending on where you stay in Kuala Lumpur, you may need to take another mode of transport to get to your hotel when you reach KL Sentral. If you are traveling in groups of 3 or more, taking the taxi might be a more practical option.
Taking the bus is always the most economical option when it comes to airport transportation. Costing just RM10 to RM12 (depending on whether you are embarking from the KLIA or LCCT terminal), the buses would take you straight to KL Sentral Station in just an hour or so. The busses are very frequent, leaving every 30 minutes or lesser and the service runs for almost 24 hours a day. There is a bus company serving the LCCT terminal that runs from 7am in the morning till 2.45 in the morning.