Don't march stacks of 150,000 men through the Alps.

Keep your army in separate corps of around 25,000 each as they move towards the enemy, until you see exactly where the Austrian army is located. (You do have cavalry scouting ahead of the main army, don't you?) Only then make the stacks converge into a single army. As soon as you've defeated the Austrians, divide them again so you stop taking attrition.
In battle, make sure your flanks are capable of choosing one of the tactics that doubles its defensive strength, such as hold, entrenched or counter-punch. In general, to achieve that you need one out of each five brigades on that flank to be light infantry or cavalry. Having artillery included as well helps you reduce the enemy's attack strength before the main combat phase.
After the war, make sure your stacks are not taking attrition, and move them back into friendly territory as soon as possible - they recover losses faster when in your core provinces.