NOTE: Since the introduction of the new Indian visa rules, visa runs are no longer possible.
Tourist visas for India are generally issued for periods of three or six months, depending on nationality. Those wishing to remain in India for longer than this have, in the past, taken a quick trip to a neighboring country to renew their visa.
Nepal used to be a favorite for this purpose. I even went there myself a few years ago. It was a simple process to get a new six month tourist visa then. However, not anymore.
The Indian Embassy in Kathmandu recently became reluctant to issue Indian tourist visas of longer than three months. Last month, a friend of mine went there hoping to get a new Indian visa. She was unsuccessful and told me that the Embassy wasn't renewing tourist visas at all. She then tried her luck in Bangkok. Fortunately, she was readily granted the visa there.
From all accounts, Thailand is currently the most reliable place to go for an Indian visa run. The other option is Sri Lanka. However, getting back-to-back visas of longer durations can be tough there. Reports of only one or three month single entry visas being issued in Sri Lanka are common.