While it is somewhat out of the way, Kep is still rather well-connected to other parts of Cambodia. You can easily get there from Phnom Penh and Kampot.
From Phnom Penh, regular bus service to Kampot typically stops in Kep. The cost of the bus trip is usually around US$5 and takes around three hours. A quicker and more flexible option is to use a private car transfer, which will run you US$25 to US$35.
From Kampot, you can catch a motodup or taxi. The fare is negotiable, around US$5 for a motodup or $10 for tuk-tuk.