Law is incredibly important, I aim for that almost to the exclusion of anything else. Also, I will make sure that every single town is building somehting each turn, EVERY turn. If this means I have a small army then fine, but just keep building stuff. Thats where the real strength of your empire forms. So up until about 250BC I have nothing but the starting level MIC's (barracks) and crappy armies composed of skirmishers and whatever melee unit I can find. Generally no cavalry unless I started with some. After that you will have considerable wealth and can start a real modernisation of your military to fund your expansion.

Also, keep in mind that recruiting a general from a Lvl 4 government is very expensive. They cost anywhere from 500mnai for the celtic footmen up to well over 1000 per turn. This will only pay off for distant and very difficult to run towns that would require massive armies to garrison. Be cheap and stingy until you build your wealth.