I need finer resolution. What do I do?

You can always select a sensor with lower noise (Analog Devices makes several models with various noise densities to suit most applications). If that is still not enough, there are two avenues to explore - bandwidth and noise density. As mentioned previously, noise floor is determined by noise density and bandwidth. Since the user has no control over a sensor's noise floor, you can try to reduce the bandwidth as much as possible for a given application. If more adjustment is required, you can average the signal from multiple sensors. As noted earlier, the noise of all Analog Devices inertial sensors is random (Gaussian). Therefore the noise of each sensor is uncorrelated with the other. So for n sensors averaged there is an improvement in noise of √n.