I've found that the best strategy in either a defensive or offensive war is this:
1) Increase your Reserve as much as possible, while slowly increasing the regular army (a division every so often).
2) As soon as (or slightly before) war is declared mobilize the reserve.
3) Pool all your troops (reserve + regular armies) into one giant stack (preferably 30 or more).
4) Send that giant stack to the border area and wait for the AI to attack you.
The AI will usually invade in stacks of maybe 6 divs at most (at least that's been my experience with France and the UK). 30 divs vs 6 will always just be a slaughter, especially when you are fighting in your country and the enemy has to also contend with massive attrition.
Just keep bouncing that one huge army up and down the line killing a division here and a division there. Eventually the AI will literrally run out of divs and manpower and its war exhaustion will go through the roof allowing for an easy white peace or allowing you to finally divy up that massive army into stacks of 10 and going on the offensive.
Using this strategy of basically making every war a defensive war, make sure you cut education to 50%, cut Crime Fighting to 0%, raise taxes to 49% accross the board, raise tarriffs and have defense spending as high as possible while maintaining a decent surplus (remember, you will be fighting this war on your territory and will lose large chunks of land temporarily, make sure you can handle the resulting loss of revenue).
This Strategy has never failed me

Successfully used against Russia (in Korea and Siberia), Prussia and France in my current Bavaria game.
I first used this strategy against the UK in my Über Republic of Krakow game and ended up destroying half the British army while only losing a handful of Divs on my part (I also lost control of most of the country, but that's a minor point

). The Brits sued for a White Peace as their War Exhaustion hit the high 70s
