Key thing that helps everyone-but-Poland for the war in the west - keep France in the fight.
You can initially do this by stalling the Blitzkrieg, and that is basically hitting the Ardennes hard. It doesn't require different organisation, tactics or equipment (though all of them would help!), it just requires a better use of resources that are available.
Even if the Battle of France is still won by the Germans, the Allies will be in a better position. More people evacuated. More experience of fighting the Germans. More German units trashed.
Assuming France has fallen, mitigate the Invasion Panic - the panicked continuation of out-dated production like the 2-pounder gun led to inadequate equipment being issued far past its time.
Pursuing Op Compass to a successful conclusion rather than turning to Greece - clears the North African theatre, saves the many many Allied troops who were killed or taken prisoner in that campaign. Also frees up resources for:
Improved preparations in the Far East - a few more experienced troops, arriving a little earlier, and the whole campaign in Malaya looks completely different. The Japanese were on a shoe-string and depending on captured supplies anyway.
If you can hold Malaya, the Burma campaign may never kick off (the troops for Burma were initially sent to Malaya IIRC) - even if it does, it's likely to go better for the Allies.
If Malaya and Burma are holding and being reinforced, a lot of the Japanese sea-lift and air support for attacking the Dutch East Indies are unavailable, so the Japanese may never take substantial oil production capacity, and their war effort will come grinding to a halt even with only limited further Allied combat.
That would limit Japanese gains to the Philippines, and perhaps some minor islands - hell, if the battle of France went more slowly, the Japanese may spend the first weeks of war fighting into IndoChina, and never reach Malaya at all.
It would also kick Italy and Germany out of North Africa 2 years earlier than OTL; might well mean the French colonies are supporting the Allied war effort all along, and could even remove Mers-el-Kebir and have the French fleet working with the RN from Sept-39 to VE day continuously.
All in all a faster and less painful victory for the Allies, and all it requires is:
1) A better Allied deployment of troops in 1940. Given the limited logistics of the Ardennes route, a defence there should be possible.
2) No redeployment of troops from Compass to Greece.
The rest should come as knock-on effects of those two.