One Division Training
One of the greatest ways to earn army XP is to delete all but one of your divisions and train it. This is very useful if you are playing a country with no imminent war, and no imminent need for manpower.
You make more XP the bigger the division is, so choose your biggest division to train. Once you have 5 XP add another battalion to your division. If you have enough time you could make your division a 50 width with support companies for maximum XP. I recommend adding infantry battalions as the training will attrition the division so you will mostly just be losing inexpensive infantry equipment. Furthermore as you develop the division use the ‘Duplicate’ feature to create any other infantry divisions you need of different sizes.
If you are wondering why one division is used, it is because when you train your divisions attrition which means they will lose equipment. By only training one division we do not lose a significant amount of equipment. Training multiple divisions usually will result in less army XP being made.
This strategy generates large amounts of XP by lend-leasing a country at war army equipment or planes. I recommend sending Infantry Equipment once with either one artillery or one support equipment monthly for army XP. For air XP I recommend sending fighters with one artillery or one support equipment monthly.
Sending an attache to a country at war is a very good way of making army XP. The only problem with this strategy is that often the AI is stubborn and will refuse to accept it. Nonetheless if you can use it you can sometimes make a lot of XP. The war support you get from sending an attache can also be very useful for chaning your Economy Law. Keep in mind you will lose the attache when the country is no longer at war.