Asynchronous low-power MACs based on preamble sampling are considered some of the most promising low-power protocols for wireless sensor networks. Although many such protocols have been suggested, they suffer from the inefficiency of essential features such as a means for preamble collision avoidance. We suggest a multiple preamble transmission scheme rather than the previous exclusive method in which only one sender can transmit a series of preambles to prevent preamble collisions and other neighbors should wait until the sender finishes all preamble and data transmissions. In the proposed scheme, the preamble interval that was stationary in previous works becomes variable. Since the sampling duration must be longer than the preamble interval, we calculate its most appropriate value from an analysis of preamble interval based on the renewal theory. Our simulation and experiment demonstrate the correctness of the analysis and the performance improvement.