More important than anything else - mobilize the Philippine Commonwealth Army alongside the National Guard in September of 1940. Historically, this was contemplated, but it was decided that this would be too provocative towards Japan - never-mind that the Two Ocean Navy Act was exponentially more of a threat towards Japan than the activation of a few light infantry divisions.
The PCA was a force built around its reserves - it had a standing strength of around 4,000 men (plus several thousand men in the Philippine Constabulary), and reserves of approximately 126,000. The training status of these forces was abysmal - only around 10% had ever participated in field training, and no exercises larger than a company level had been carried out. The reserve officer corps was in especially dire shape, with most officers having undergone a crash three to five month part-time course of instruction, with their subsequent reserve officer rank (ranging from third lieutenant to major) determined by their social rank in civilian life.
By the time the PCA was called to the colors in late 1941, it was too late - the first wave of mobilized reservists only received a few weeks of training, and the third and final wave was activated concurrently to the Japanese invasion. Simply having over a year to train, hold field exercises up to a divisional level, and sort out the worst reserve officers while improving skills in the best that remain would result in a far more capable force come December 1941, even without significant improvements in arms and equipment.
Also - the US government had been holding several hundred million dollars derived from excise taxes in trust for the Philippines, intended to handed over to the newly independent government in 1946 to smooth over the expected budget troubles that would come separation from the US. In the late thirties, the Commonwealth government made several requests to tap into this fund to help pay for national defense efforts, but were rebuffed each time. Allowing them access will go a long way to helping the PCA.
With what? The US Army of the time barely had enough to meet its own requirements upon mobilization - and after the British withdrawal from continental Europe, they didn't have even that. The US hastily rushed the arms and equipment to organize ten infantry divisions to Britain, which left a gaping hole in their stocks which couldn't be readily filled.
