The single best way to begin in this area is to raise your awareness of others:
When conducting social engineering attacks, being very aware of this is part of understanding your target's reactions. Courses in social awareness, knowing your target, Neurolinguistic Programming and even sales training all build in elements of this, but it all comes down to practice and time - you can only get so far with theory.
Once you gain an awareness of what to look for, you will be able to look in a mirror to see what your face does.
And, disappointingly, you are likely to find at first that your microexpressions aren't controllable at all!
A suggested way to progress is to try and imagine basic emotions (like joy, sadness, terror etc) and seeing how they affect your expression. See what happens when you hear something sad but are thinking about something happy - that disconnect is what can happen when you are confronted by a target - your conscious mind is concentrating on not getting caught, which is different to the response of someone authorised to be there. So try and become adept at imagining and feeling the emotions of whatever character you should be.
Associated with this, being relaxed about the scenario should also reduce the magnitude of microexpressions, which makes them harder to pick up. Again, this will come with practice. I still find social engineering to be a challenge, as I'm often thinking about possible outcomes of any confrontation, rather than just going with the flow, knowing the scenario inside and out.
tl;dr - practice, be relaxed, gain familiarity with the scenario, and practice