For a start, the Japanese ditch their racial propaganda and outlook. No "superior to the Chinese" crap, no "saving China from itself", none of that. This means too that all the IJA atrocities against the Chinese are butterflied (no Unit 731, no Nanjing Massacre).
Second, the Japanese are a lot savvier, playing up the discontent within China. On the eve of the Japanese invasion, the KMT barely controlled half of China- yes, it was the valuable heartland with the most people and the most natural resources, but looking on a map, a huge portion of the country's landmass was out of their control. Obviously, those warlords had armies; they fought with Chiang before the war, and one of the main reasons they sided with him during it was that Japan put its foot in it with Nanjing et al. But if Tokyo doesn't portray itself as out to destroy China, you think Long Yun or Sheng Shicai won't be happy to hang Chiang out to dry? They don't have to do anything much- just work out a modus vivendi with Tokyo and stay home while Chiang goes off to fight. Will it make a war-winning difference? No, but it will mean Chiang (a, has less cannon-fodder to use up, (b, anyone in the KMT and his army has places to bolt to if they want to get out, and (c, one or two of the warlords might actually start fighting the KMT if Japan gets very lucky. Of course, Japan can always backstab the warlords later on, but that's not the point.
Third, establish better relations with the West. Without getting into butterflies too much, don't be quite so hostile to Western interests (up to and including not grabbing French Indochina if you're really serious), and play up the dictatorial nature of Chiang's government. Do your best to make Wang Jingwei's government seem quasi-legitimate (it's a stretch, I know). All this to say: get the flow of aid to Chiang cut off, or better yet, don't start it in the first place. According to Jonathan Fenby's The Penguin History of Modern China (an excellent read, by the way, can't recommend it enough!), the West loaned the Nationalists over half a billion during the war (1). That's in US currency, not worthless Nationalist banknotes. From 1943 onwards, the Allies were committing to ship Chiang some 5,000 tonnes of supplies a month. I could go on; my point is that if Japan can, through insanely good diplomatic manoeuvring, get this tap turned off, they'll be in a much better position.
Fourth, establish stronger relations with the Soviet Union. Play down the anti-Communist rhetoric. Why is this important, given that the USSR didn't intervene in the war until it was too late anyhow? Well, this allows you to do a couple of things. If the USSR isn't funnelling fighters to Chiang (decent fighters, too), that helps. Good aircraft are just a microcosm, of course; if aid from the West and aid from the USSR is cut off, Chiang will be up a certain creek. But wait, there's more. Xinjiang and Mongolia are both in Stalin's pocket at this point. If Moscow can send weapons to Sheng Shicai or Khorloogiin Choibalsan (whew!), and they start pushing their autonomy and claims on Inner Mongolia respectively, that'll help.
Better still, a modus vivendi with the USSR enables Japan to co-operate, at least tacitly, with Mao. Now, the Chairman might not listen, but if he's not establishing bases behind Japan's lines and forcing Tokyo to waste men eliminating them, that can only help.
(1) Fenby, Jonathan, The Penguin History of Modern China (2008), pg 304