Please be aware that a one-year multiple entry non-immigrant B visa is valid for a period of 12 months. Each entry is valid for 90 days. After each entry you are required to leave the country via any border checkpoint to secure another 90-day entry stamp. This is simply a stamp, no visits to a Thai consulate nor any fees are required. A short bus trip to Cambodia is the easiest route, but any border checkpoint will be fine.