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 Radius of
their brigade leader, and brigade leaders within the 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.
Don't squander your high-value artillery and cavalry units
Both are worth more per-strength-point than infantry. Prevent your
artillery from being overrun, and use your cavalry wisely. Don't
use cavalry to attack enemy units; better on the defense, or as
scouts. In the same note, protect all potentially vulnerable
artillery units with infantry units, as their defensive strength
in melee is minimal (easily killed by enemy infantry if not
Keep your units in supply, especially attacking ones.
Keep your supply wagons several hexes behind the front line, but
still close enough to automatically resupply a front-line unit
that runs low on (or out of) ammo. Each supply wagon has a
"resupply" range of 5 hexes.
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.
Outflank your opponent!
One of the major tactics in the American Civil War, and it carries
thru quite well with BGG, is the importance of outflanking your
opponent. Especially when attacking, attempt to "stretch" your
lines to overlap one flank or the other (or both?!) of your enemy,
and hit those units with enfilading fire (a hefty +2 combat shift
in your favor).