Once you get to a certain point of territory held just about no one is going to be friendly with you due to territorial expansion penalty in diplomacy. Speaking of which makes sure you nab just about every single trade agreement you can as early as you can.
Money making for myself is never a problem really, I arguably do better at that than actual fighting bit in this game. I always improve my roads as soon as possible and during early game concentrate on philosophy and military trees. Wars are an expensive affair so it's important not to overstretch yourself, this is the reason I try to limit myself to not fighting land battles on more than 2 theaters at a time and leave Europe until I have a situation in North America and India well in hand. Fighting in Europe is expensive and relentless once you cross the English channel.
In terms of upgrading all buildings that's actually not the best tactic and at one point in the game you will find yourself having to prioritise where you invest the money. If you have two farms or towns and money is short, only pick the one with better yield and wealth. In terms of military buildings for example, do you really need two expensive military barracks close to each other when you can easily get away with one?
Another VERY expensive aspect of this game is the navy, so rather uncharacteristically for the British Empire I leave that part of the armed forces undeveloped on the research tree and small in size. I'm careful about picking my battles and only engage easy targets, other than that just use it for piracy and defending the odd port blockade on the British Isles. I don't even bother with a navy in the North America and India as that can bog you down financially and ruin you, I'd take loss of trade revenue over having to build, maintain and repair a large navy for little benefit any day of the week. For very same reason I don't bother with any trade theaters, it's just too expensive having to maintain the navy in those regions to protect your trade ships.
Do also check your cabinet of ministers as someone mentioned, bin anyone below three stars immediately and replace the third star ones when you can too.
I find that places like India or major European cities can be a pain in terms of rebellion and religious unrest. In India you may find that after capturing a certain big city you may find an awkward combination of occupation penalty and religious unrest that isn't quite big enough to cause a full on rebellion but big enough having to keep a large garrison there. What I actually prefer to do once I capture a region there (or other troublesome regions) is replenish my army ASAP but don't keep them in a city but just outside with a small garrison inside. This does cause a fully blown rebellion in a couple of turns but which point my main stack is fully replenished and can normally squash it in one turns (two at worst). You'll find that after this the penalties for unrest are far lower and you no longer have to keep a big garrison there in less than a half a dozen turns so works out cheaper in the long run. Just leave a couple militia units there without the need to issue a tax cut and your main army is free to march on.
