Visa- You will need a visa. Start by visiting the website of Cambodia's Ministry of Foreign Affairs for useful tourist information, including a link for the E-Visa application. E-Visa site sometimes crashes but eventually is up and running again. The visa costs $37 per person.
Immunization- The Center for Disease Control (CDC) recommends receiving Hepatitis A and Typhoid vaccines, and making sure your Tetanus shot is up to date. Since anti-typhoid medication is often cheaper than the vaccine, consult with your doctor whether that is a better option for you. As this is seldom covered by the insurance, those living in the U.S. may find it to be cheaper to get these vaccines in the CVS Minute Clinic.
Don't Exchange Any Money- Cambodia predominately uses U.S. Dollar (USD) instead of Cambodian Riel. All the prices for tours, tickets, food, and souvenirs/clothing, as well as all bargaining, is all in USD. Cambodians seem to prefer using dollars and are almost confused when you want to pay with Cambodian Riel. Don't make the mistake we made when we exchanged $200 at the airport- that was money wasted as the exchange rate that the sellers used to calculate the price of an item quoted in USD was always lower than the one we paid.
Bring Lots of $1 Bills- This will be so useful and convenient for you. From tipping the staff and paying for cotton T-shirts in front of the temples, to taking the tuk-tuks and buying water and fruit, having small bills ($1 and $5) will make your life easier and will be much appreciated by all the sellers. Plus having small bills helps during price negotiations.
Have Enough Cash for Angkor Tickets- Believe it or not, this is CASH ONLY, they DO NOT ACCEPT CREDIT CARDS. One day pass is $37, while the popular three-day pass is $62 per person.
Internet- If you have T-Mobile from the U.S. visiting Cambodia is super easy internet wise as Cambodia is one of the countries covered by T-Mobile One Plan, which means unlimited 2G data (just be sure to Disable LTE roaming on your phone). Otherwise, many cafes and restaurants that cater to foreigners have free wifi.
Electricity- Europeans and Americans will both be very happy with electrical outlets in Cambodia, as they are designed to accept both types of plugs. The electricity however is 230V, so if your devices only accept 110V, you will need a voltage converter/transformer.
Safety- Based on the warnings we got, Siem Reap seems to be safer than Phnom Penh. Either way, use common sense- don't talk on the phone while in tuk-tuk if it can be easily snatched, keep your purse close to you, lock your valuables, etc. As for food and drinks, we avoided ice and unpeeled fruits and vegetables, but ate in numerous restaurants, some very modest, and have never gotten sick. We did not taste any street food though.
Transportation- Contact your hotel ahead of time and ask them to organize airport pick up. In Siem Reap transportation from and to the airport was free of charge, while in Phnom Penh it cost around $18 each way. For local transportation, tuk-tuks are the best way to go- they should be between $2-$4 around Siem Reap, little more in Phnom Penh. We didn't bargain with them (as we heard how hard of a job it is), but we did ask about the price before getting in to avoid any disputes later on.
Shopping- Those who love bargaining will have a blast. Just remember that the sellers live from the sales of these goods, so don't let your desire to hone your bargaining skills put the sellers desperate to sell out of business. You can buy cheap souvenirs in local markets everywhere. You can also visit Artisans Angkor stores in Siem Reap and at all airports in Cambodia for beautiful Cambodian arts and crafts- everything is hand-made, and it is rather pricey as the proceeds from sales help pay decent wages and help local communities.