Abstract: Provided herein is a clamp and method of use thereof for agitating, mixing, or shaking a fluid container using a mechanical agitator, such as a hand motion shaker. The clamp includes: at least one grip adapted to attach to at least a portion of a fluid container; at least one base comprising a high friction portion configured to contact a bottom portion of the fluid container; a supporting member extending between the grip and the base; and a mounting structure for connecting the support member to a mechanical agitator. The fluid container may be a flask including an elongated narrow neck portion extending from a bulbous portion, which has a larger inner diameter than the narrow neck portion. A method of clamping a fluid container and an assembly including a clamp and a mechanical agitator are also provided.