Here's a useful tip we've found for creating the best possible crema while using the Nanopresso and Minipresso. Simply rest the machine on top of the cup you're using. This allows the espresso to flow deeper into the cup, while not splashing on the top and causing disruption / large bubbles on the top of your crema. The smoother your crema is, the better the mouthfeel. This creates a better overall espresso experience and gives you a consistent sip which allows you to better discern the different and subtle tasting notes of a particular roast's crema.
Be sure to dial in your grind. The finer the grind, the more pressure you'll be able to create. The more pressure that builds, the better the extraction and the finer the crema.
It's always important to use as hot of water as possible with your Nano or Mini when attempting to achieve the best possible shot. Hot water will result in the best extraction, and a better extraction will give a golden brown hue to your crema.