Watch your organizations:

Use the "Show Organization" tool box button to keep track of the various units to keep them within their respective command radius. If at all
possible, keep regiments within the Command adius
of their brigade leader, and brigade leaders within he command radius of their divisional leader. This helps prevent undue routing & disrupting, and aids in rallying.

Place your artillery units carefully:

Use the "Check LOS" tool box button to "test" the lines of sight from various vantage points. The more you have to move an artillery unit the less use it can be.

Keep your reinforcements in mind:

Use the "Reinforce" pop-down menu to check on the arrival time and location of any reinforcements you're scheduled to receive (if any!). The proper
use of your reinforcements, may be the key to winning or losing.

Keep a reserve:

If at all possible, keep a reserve, whether on offense or defense. It just makes good common sense to have a readily available force to "plug" the gap created by an unexpected rout, or to have some fresh troops to throw into that breakthrough.

Watch your units facing!:

Facing in Gettysburg is critical (for infantry,
cavalry and artillery). Always check the facing/formation icon for each unit as you select it (in the Unit List's icon display area). More than once I've had enfilade shots at opponent's units because they did not have them properly positioned.