First and most importantly, a nation best supports its active duty military by sending those troops into harm's way only when essential for protecting vital national interests. Vital national interests can be difficult to define. Unfortunately, politicians often employ the phrase without bothering to define it. Vital national interests denote interests that, if not protected, jeopardize a nation's continued existence. This definition generally precludes citing economic factors to justify deploying troops. For example, the repeated US interventions in Latin America during the nineteenth and twentieth centuries known as the Banana Wars were all wrong because they benefitted US corporations but were never a response to an existential threat against the US. Protecting or improving the economic well-being of some citizens or businesses is insufficient to justify military missions that pose an existential risk to military personnel, i.e., expecting others to be willing to die to protect my standard of living is morally wrong. Using military force in that manner reduces military personnel to a means to an end that lacks lasting value. This restraint on the use of military force is another way of expressing Just War Theory, the historic Christian perspective on when using military force is morally justifiable. Incidentally, failing to aid another nation or people when it faces an existential threat would also satisfy the test I propose for when using military force is morally justifiable. Left unchecked, evil that destroys another nation or people, as exemplified by Nazi Germany in WWII, will continue to pursue its evil goals until stopped, eventually posing an existential threat to the globe. Stopping genuine existential threats early prevents unnecessary harm to life. Conversely, mislabeling something as an existential threat will often result in increased harm, as occurred with the misguided US invasions and conquests of Afghanistan and Iraq.
Second, a nation supports its active duty personnel by adequately compensating them for their service and the risks they may face. The current military pay system falls short of that standard. We pay junior personnel too little. We promote too many officers, creating bureaucratic bloat and incompetence, because of the up-or-out promotion system. The all-or-nothing retirement system that requires a service member to serve twenty years or leave with no retirement benefit both exploits those who serve a lesser amount of time and needs adjusting to reflect longer life expectancies. Moves to reduce health benefits for military personnel and veterans shift compensation in the wrong direction: military service in peace and combat is hazardous and a nation has a moral obligation to care for those who serve.
Third, a nation best supports its troops when everyone serves (cf. my Ethical Musings' post, Memorial Day). A program of national service in which everyone, regardless of gender or ability, serves would renew commitment to the nation and represent a major investment in the nation's future. I would propose that everyone serve for two years, post-high school. Compensation would be set at the minimum wage. Education deferments would be freely available, but would be just that, a deferment. Upon completion of her/his education, the person would still have the obligation to serve for a year at minimum wage. The advantage in granting deferments is that the nation could reap the benefits of the person's education without having to pay additional compensation, e.g., a doctor would serve as a doctor, but receive the same compensation as someone who served immediately following high school. There is no exploitation in this: those seeking a deferment would know the terms. A parental leave provision would function similar to educational deferments: new parents could take two months off and then return to complete their service, earning the same compensation as everyone else completing national service. Assignments could reflect personal preference, abilities, education, and the needs of the nation. The more professionalized the military becomes, with increasing numbers of career personnel being part of multi-generational military families, the weaker the connection between a nation and its armed forces becomes. The weaker that connection, the more prone a nation is to misuse and to fail to support properly its military.