Shanti a METV is fairly straightforward. The 'clock' on the visa starts the day it issued so do not get until near when you fly to Thailand. The visa gives you a six month window from date of issue where you can enter Thailand as often as you like. Each time you will get stamped in for 60 days. By timing your last return just before the end of the visa period and then going to immigration and paying 1900 baht to extend by 30 days you can have a good long stay albeit with a few border bounces and things.
Ian all good advice except maybe lunch! I only had a short time I could stay in Tachilek and did not see anywhere I wanted to have lunch so I zipped it back to Mae Sai where there were many options.
Tod I have no doubt that your information is correct. Having said that one had to wonder how there could be so many horrible Thai drivers. I have ridden with many Thai friends that are perfectly good drivers but have also seen so many outrageous drivers. I myself will never try to drive in Thailand.
Robert's advice is correct. I had a similar experience to yours. I stressed about whether to spend the time and money on a 30 day extension. In the end I showed up at immigration with a 500 baht note but did not need it.
Duncan your comments about Tachilek gave me a chuckle. I have only been there once and only stayed long enough for a few pics and a short walk about. What struck me was that even though you have only crossed a short bridge you can tell very easily and immediately that you are in a different country.