When you arrive at the location you park your car then go by 4x4 to the top of the mountain. On the way you will pass soldiers. They are "protecting" the Temple's location as there is an on going dispute with Thailand as to ownership of the land. My tour guide was Rin Ra from Siem Reap [www.cambodiaincomfort.com] and is highly recommended.

Our hotel hooked us up with a driver who turned out to be half driver, half guide. We did a day trip from Siem Reap. It's a long, long day ...12 hours. The estimates are 2-4 hours one way, be assured it is 4 hours each way. Our driver was excellent, drove carefully and as fast as the conditions permitted. The ride out there and back, the temple itself are fascinating. The place where you show your passport and purchase a ride to the top is kinda creepy as is the entryway to the temple complex. There are soldiers living all along the way to the temple and it seemed in the temple complex itself. We read that people brought food for the soldiers whose monthly pay is paltry (about $50/mo). With our driver's help, we purchased noodles and cookies, putting them in biodegradable bags. We gave to children, women and soldiers on the way to the top. People seemed appreciative. The older kids sharing with the younger. The temple itself is beautiful and close to empty. We did not take a guide for the temple nor was there one in the offing at the top. Look up this site on Wikipedia before you go, and read its history from ancient times to the present day including at the end of the Khmer Rouge era. Was there a temple fee? It cost either $25/truck or $10 to go up on a motorbike. The ride is much less precarious than some of the posts led me to believe. The $25 charge is quite high for the very short trip (10 minutes, only 2 minutes of true four-wheel drive) to the top. For the money and the time traveling, we prefer Banteay Srea, much closer and a beautifully preserved temple in a forest setting.

After an early drive from Siem Reap and a stop to visit Kor Ker, we arrived at Preah Vihear Boutique Hotel in the afternoon. The Hotel is about 30min drive away to the temple. We left the hotel at 3pm. Do note that the last admission is 4pm.
My guide Mr Chenla (www.angkortempleguide.com) arranged for his relative to take us up the temple. The ticket booth is located a short distance from the entrance to the site. You have to present your passport (or a photocopy of it) to get the ticket. The ticket itself is free of charge but you will need to pay for the transport up the hill. You can choose between motorcycle or 4X4 pickup and these are available at the ticket office. I was told the fees could be between US$10-25.
My guide's uncle is a soldier stationed there and he took us up the hill in his battered pickup. The ride up the super-steep hill was an experience itself. Nothing dangerous nor spectacular but not your usual tourist experience.
There were Cambodian soldiers with AK47s but the mood was very relaxed and friendly. The soldiers were probably more amused to see the occasional tourists to break their routine and boredom.
One of the soldiers came over and became our guide. He spoke in Khmer and Mr Chenla translated to us in English. He pointed out that the temple was a proud symbol of the old Cambodian currency and showed us an old Cambodian currency note. He explained many things as well as pointed out the best spots of the temple. He even described the brief artillery battle with Thai forces and pointed out where the shells landed and the spot where one of his comrades was killed.
Another soldier offered his binoculars to us and pointed out Thai military post. Even without the binoculars, we could see the Thai flag and could hear Thai pop songs quite clearly. Our guide gave them some tips for their assistance.
The temple itself was magnificent and there were just a few tourists wandering around, it was serene. At the end of the ruins, the sheer cliff was a sight to behold. There was no safety railings or ropes, so please be careful while enjoying the sight. My wife and I watched the sun set and it was surreal. Totally worth our long journey and expensive stay at the hotel (Preah Vihear Boutique).
At the end, we took the beat up pickup truck down the steep hill again and paid the soldier US$20 for the ride. After a night stay, we drove back to Siem Reap. The roads to Preah Vihear are relatively good and it is just a matter of time before it is easier to access and the temple be spruced up to be more tourist-friendly. Till then, it is still possible to enjoy the tranquility of this awesome ancient temple.