To clear a small misconception you might have, the reformation of the Roman army from militia to a standing professional army occurred a few centuries before Augustus's reign. The first sign of change was with the Marian Reformations in the second century B.C., though many more changes were needed to make the Roman legions into the disciplined and logistical force that most people know them as today. In terms of how it was promoted for enlistment, men were generally enticed by promises of glory, loot, and the honour of giving service to Rome. Apart from the spoils of war and pensions, legionaries grew accustomed to being rewarded with a plot of land after their 25 year service, either in recently conquered lands or back home. Direct enlistment into the legions was, between the first Marian Reforms and the reign of Augustus, limited to Roman citizens, though non-citizens were often willing to join as auxiliaries. Auxiliaries served in both pivotal and non-conventional roles, such as as skirmishers, archers, calvary, scouts- roles generally not undertaken by Roman troops. In the case of auxiliaries, the main incentive to joining the Roman army was for the promise of benefits, the most valuable being that of Roman citizenship for theirselves and their offspring.

Source:Parker, H.M.D., The Roman Legions. (1956)