At least, it is not so difficult. Most of foreign travellers prefer to drop at Phuket and taking a private transfer to Khao Lak.  This tranquil beach nestled on the beautiful waters of the Andaman can be reached by air, by bus, by car or by train.  A flight to Phuket Airport, followed by a road trip of a further 80 kms to Takuapa town and within an hour you will be in Khao Lak.  Most of resorts have arrange those private transfer for you.  In case you don't have: Island Tour Co., Ltd at Phuket: http://www.tour-phuket.com is the one I recommend.

By Air:

    Many airlines now fly directly to Phuket from several countries, a stopover a day in Bangkok is unnecessary.  However, in case of departure from Bangkok, you may catch a domestic flight by Thai Airways which fly daily to Phuket.

Thai Airways: 
Bangkok to Phuket:
HQ: +(662)513-0121
Donmuang: +(662)535-2081-2
Phuket: +(6676)25-8237

Phuket to Bangkok:

Phuket Airlines:
HQ: +(662) 679-9395
Donmuang: +(662) 5356695-7
Phuket: +(6676) 220187, 35-1337-8

By Bus:

    From the Southern Bus Terminal to Khao Lak everyday
        - Bangkok Office: +(662) 435-1199, +(662) 434-7192
        - Phuket Central Tour: +(662) 435-5016, +(662) 435-7428
        - Liknite Tour: +(662) 435-5018-9

    Bus from Bangkok to Khao Lak is relatively punctual and inexpensive. There are ordinary and air-conditioned buses.  Food and beverages are served on the air-conditioned buses.  Bueses also make scheduled stops at local restuarants for about half an hour, to fill your stomach and take a short break.

Bus runs in Phang-nga

Bus runs in nearby provinces

By Car:
    From Bangkok to Chumporn along Highway No 41 about 500 kms, turn right on Highway 401, passing Phanom-Suratthani and to Takuapa - Phang-nga on Highway No 4 to Khao Lak.

By Train:
    You may start from Hualumphong Train Station in Bangkok to Suratthani Province for 12 hours, then catch a bus to Khao Lak.  Personally, we don't recommend you to choose this mode of travel.