Many of the advantages of microservices can be achieved by refactoring your monolith code to be less monolithic.
I would suggest using functional styles wherever possible, plenty of isolated unit testable code, and a hexagonal architecture http://alistair.cockburn.us/Hexagonal+architecture that pushes all the I/O, mutation, side effects, etc. to the very boundary of your code. Also see Gary Bernhardt's "Boundaries" talk for more interesting thought in that vein https://www.youtube.com/watch?v=yTkzNHF6rMs
I would suggest using functional styles wherever possible, plenty of isolated unit testable code, and a hexagonal architecture http://alistair.cockburn.us/Hexagonal+architecture that pushes all the I/O, mutation, side effects, etc. to the very boundary of your code. Also see Gary Bernhardt's "Boundaries" talk for more interesting thought in that vein https://www.youtube.com/watch?v=yTkzNHF6rMs