Don't know how many people try to use a checkerboard formation, it really, REALLY seems to work, really flexible. This works really for any non-phalanx faction, but especially Rome. I go
----Skirmisher--Skirmisher
--------Hastati--------Hastati
Principe--------Principe-------Principe
------------General---------------
Sometimes I have archers behind the spaces between the Principes. After Marian Reforms, I use Legionary Cohort instead of Hastati and Principes, and Auxilia behind the Principes, with one archer behind them. Depending on battle circumstances, I may move the Auxilia to the front or flanks to deal with Cav. I rarely get much Triarii before the reforms, and Praetorian Cohort do not have as good stamina as Legionary Cohort. I will usually also have one unit of Roman Cav on one rear flank, one Legionary Cav on the other rear flank, to swing around and nail the enemy as needed.