In the Napoleonic era, massed batteries rule. The early cannon in the game though are not quite that effective, so the best way to use them is to deploy them in the gaps between infantry units (pretty much as done historically, in fact).

I like to always deploy in two lines, with the cavalry behind in a third line (usually out on the flanks), ready to charge out and wreck havoc. Second line is never set to fire at will... you need to manually control that, unless you'd like to have them shoot your first line in the back.

If the enemy only attacks one end of your line, swing your other troops round to enfilade them at the double.

Some light infantry thrown out in front of your army can be very effective. They get to fire first at long range (causing the enemy attack to become more chaotic as they try to deploy) and can then fall back on your main body of troops (never let them get into a firefight).