Here is a link to some travel advice for all visitors to Cambodia.
1) Visas are available on arrival (also known as the "VOA"), cost US$30 and take about 20-30 minutes to process. One can also apply for the visa online ("E-visa") at Note that at some border crossings they do not accept the e-visa, be sure to check which crossings accept them on the site prior to purchasing the e-visa. And, if you are going by cruise ship up the Mekong from Vietnam, some companies will require you to have your visa in advance, in which case you have to send your passport, application and payment to the Embassy in Washington, DC .
2) While in Siem Reap or Phnom Penh you can gain a lot by hiring one tuk tuk driver during your whole stay.This is one way to build a rapport, to get good continuous service and to remove some of the hassle factor in having to flag down a tuk tuk.
3) Booking internal travel from Siem Reap to other centres such as Battambang or Phnom Penh is generally easy with most every hotel and guest house having good local knowledge. Ask your hotel/guest house manager and they will be sure to help. Bus services are pretty good (modern coaches)and the boat ride is another option, but do know that safety standards are not up to par with western standards..
4) If you are really stuck for internal travel - for example buses are booked, the boat isn't operating (during the driest of the dry season) then consider hiring a taxi. Siem Reap to Phnom Penh is around US$70 and split two or three ways as a group makes for an economical option. However, fasten your safety belts! Taxi drivers have quite modern Toyotas (no problem there) but drive aggressively, with one hand poised on the horn for a good proportion of the journey - as they navigate roads that are populated by cyclists, cattle, motorbikes, pedestrians and huge army trucks. A memorable journey is guaranteed - not for the faint-hearted.