My general strategy is to take your entire population and fight another country as soon as possible to increase your population, since when your population is low, your options are limited. Leaving behind no people makes the original country vulnerable, but that doesn't matter since you don't lose anything. Once your have significant population, start conquering countries, paying close attention to country borders. Once a country only has one target, take the entire population, since leaving no people behind is irrelevant since no one can attack it.

Try to avoid using non-family member princes if possible, since they seem to rebel at annoying times. Also try to leave any country that can be attacked under direct control, since your princes use the same bad AI that the computer uses and will be unlikely to properly defend itself, or may attack at an improper time.

When increasing your lord's stats, focus on leadership (important for training troops and going to war), judgement (trading), and planning (assigning population and battle units). The other stats won't be a major concern, since if you go to war and win, you have a net gain in charm (10), body (20), and battle (5), whereas the other stats will go down or have a small increase that will be used up in non-war commands.

When at the end of the game, use a prince to battle at a minimum for the last battle, since the results won't matter as long as they win.