To have your troopers gain more experience after each battle, go to the "stta\units" directory and open the st_db_attr file in notepad. Go to line 26. It should start out as:

26 1 0 0 15 0 0 1...

Change this line to

26 1 0 0 200 0 0 1...

You can try other values besides 200 to tweak the experience growth of your troopers.