Thailand is famous for foreigners to stay on an extended vacation.
  However, tourist visas from western nations such as the United States, Canada, the United Kingdom, and Australia require you to leave the country every 60-90 days.
There are several reasons for this both political and economical, but that's another subject.
I did the trip from Bangkok since that is the most central and populated location.
  Bangkok, among all tourist areas offer services that tout "visa runs", mostly at tour agency and also can be found at many hotels.
The prices charged are very well inflated by Thai standards, however it worth it to many people as these services complete everything for you.
You don't even need to physically see an immigration officer! You simply fill out the forms and a person from the agency takes them along with your passport to the border and gets you your stamps.
I've always found it difficult have my passport handled by another person, especially a complete stranger from another country.
However, there are countless agencies that provide this service and I've known may people to use these services with no problems.
They are generally a safe service to use.
Because I enjoy frugality, I wanted to do this the cheapest way possible.
Here is what I did and how much I spent coming from Bangkok.
 

1. Took the subway to Mo Chit station.
   This will cost anywhere from 15-40B depending where you are coming from.
    
2. Take a taxi from Mo Chit Station to the Northern Bus Terminal.
   50B.
   
3. Get a bus ticket to Aranyapathet for 400B, round-trip.
   
4. The bus stops at the Aranyapathet Bus Station which is about 6kms from the border.
   You can walk, but it better to take a tuk-tuk for 50B.
   
5. This step is important.
   You will have Cambodian men approach you near the border multiple times to assist in the Visa processing.
   YOU DO NOT NEED TO DO THIS.
   They will be extremely friendly and hospitable.
   They will have an official-looking application form.
     You can accept their offer if you choose, but you pay more and plus you have a complete stranger handle your passport.
   Politely decline and keep moving towards the Thai Immigration office.
   Once you arrive, line up in the Foreign Passport area.
   Make sure you have your Departure Card.
   They have them there so don't worry if you don't have yours.
   Get your departure stamp and exit the office.
   There is no departure fee.
   
6. Next you will need a Cambodian visa.
   This is one of things the guys offering assistance will do.
   Do this yourself with ease.
   Once you exit the Thai Immigration office, go to the right to the Office of the International Border Check Point of Poipet.
   You will walk into a large fairly empty-looking room with Cambodian government officers in uniform.
   You will fill out the visa application and submit it to the window.
   There will be officers to help you with this.
   However, they will ask for a larger fee than is necessary.
   It is clearly posted that the fee is $20USD.
   Politely insist that you will only pay that amount.
   If you don't have US dollars than it is 1000B, the price it is at the Cambodian Consulate in Bangkok.
   They will ask if you have medical insurance or say there is a new processing fee.
   Not true.
   These officers while looking trustworthy only earn about $30USD a month and will try to extort money from foreigners.
   But remember to always be polite in your dealings.
     Also, there is a passport photo required on the application.
   They will ask for 100B for the photo to be taken.
   I paid it since I did not have a photo, but they never took my photo.
   They processed my visa and sent me on my way! The photo is not necessary but I would have one so they can't call you on it.
   
7. Next I proceeded to the Cambodian Immigration Office, which is about 500m down the road.
   You have technically left Thailand now.
   There are several casinos and many duty-free vendors along this 5 minute walk.
   You will encounter more men trying to assist your processing and children and beggars.
   Keep walking.
   Once you arrive at the Cambodia Immigration, you will receive an Arrival form to complete.
   This time, you will have the Cambodian officers offer to process this for you for a fee.
   Again politely decline and stand in line.
   There is no fee.
   
8. Once you receive your stamp, exit the office and you are officially in Cambodia.
   There is not much to do in the small town of Poipet, but things are found even cheaper than Thailand.
   
9. You can now turn around and go back into Thailand.
   The Departure post is right across the street.
   Fill out your Departure form which they have stapled into your passport.
   Stand in line and have your passport stamped once again.
   There is no departure fee.
   
10. Walk back down the street past the casinos to the Thai Immigration Office.
    It will be on the opposite side of the street where you came in.
    By know you should know to ignore anyone offering to help you.
      Fill out your arrival form available at the office and get processed again.
    
11. Leave the office and you are now in Thailand again.
    Your visa renewal is now complete.
    Take a tuk-tuk for 50B back to the bus station and head back to Bangkok.
    

Here is a summary of the costs: Subway/taxi to the Northern Bus Terminal                                                       90B Round-Trip Bus Ticket to Aranyapathet                                                         400B Tuk-Tuk from Aranyapathet to Border                                                              50B Cambodian Visa                                                                                          600B ($20USD) Tuk-Tuk back to Aranyapathet Bus Station                                                      50B Northern Bus Station to your final location in Bangkok                                     90B Food & Drink (personal choice)                                                                    100B                                                                                                               1380B ($43USD) Provided your timing coincides with the bus schedule, your entire trip should average 9 hours.