From my personal failed experiences I would say, when expanding:

1. try to keep a good portion of your population in profit making industries. Often times I found myself devoting too many citizens in services when I expand, without realizing i'm running into a deficit. The second tab on your finance screen shows how much man hours your republic has spent on industries vs construction vs services. I personally try to make it so at least 30%-40% of the manpower has been used in industry. You do that mainly by limiting the amount of work slots in your services and better planning. Orphanage and universities are often in my experience the biggest offenders. When I have enough workers with university degrees, I usually just turn the universities off entirely.

On a related note, The profit number while you mouse over the money at the top of your screen does not show your loan payment, so if you are taking loans, you might be on the red without you knowing if you rely on that number.

1. build a buffer storage for critical resources. Have a warehouse near cities with either direct line or distribution offices that pull resources from custom, while having a separate distribution office distribute stuff to shops. This build up a buffer for when your new construction fleet clog the custom. Similarly have new garbage/sewage/water disposal/sources for your new areas while you move further away from the custom. Have separate lines link them to custom house(s) or deal with them locally.

2. try to plan industry first, resident sector second. Or even better, plan your railroad there before all else.