"The first secret of long life", says ancient wisdom, "is don't die."
Business is a long-term game that is only "won" over a long series of moves. The vast majority of the time, I see companies run out of time or energy before figuring out the flywheel that will keep them going.
First, keep a ledger of expenses - including your time. The Slicing Pie framework gives a decent structure for how to account for cash, in-kind contributions, and time. The means of accounting could be as simple as a Google Sheet. This habit will help you manage how co-founders and early contributors participate as well. But knowing what you spend is the first step toward controlling your burn.
Second, keep your day job - or get one. Until the business can support you full-time with income, you are the single highest source of burn. Worse, spending "free" time on the business distorts the efforts you put in. As an investor in your own business, you should want as little of you to work on it as possible - you are expensive! By having an outside income, you extend your runway, giving your business more time to find its footing. Not all work is alike - pick that which you are good at, will find some reprieve from the stresses of startup up, and will be paid fairly for.
Third, talk to customers. Startup School at Y Combinator has a cool little field in their weekly report where they ask how many customers you have talked to. They call you out if the number is fewer than five. They celebrate if five or more. This seems about right to me - a consistent habit of talking to a customer every business day will generate compounding returns.
In most scenarios, time talking to customers is far more valuable than building or just about anything else you can do. It is also usually cheap financially - get on that Zoom or call. And it gives tremendous leverage on the time and money that you do spend building or learning technical topics.
Getting a coach or an accountability partner helps a lot in spending money well. Someone who wants you to succeed but has an outside perspective will give useful feedback. Half the time the real value is in having to say what you have been doing out loud. Listening to your own words can be enlightening. As part of this, I encourage companies I work with to send a weekly report of their activities and learnings. Sadly, not everyone does this. Those who do not get more from their meetings (because it was briefed) and see their progress over time as reports accumulate.
In his classic Think and Grow Rich, Napoleon Hill advocated the mastermind. A mastermind is a handful of people all dedicated to each other's success. They commit a limited amount of time to work on each others' businesses in a confidential setting. The successes I have experienced are 60 minutes meeting weekly.
This mutual support differentiates it from a coaching relationship. Working on the businesses of your fellow entrepreneurs gives you insight into your own. Hill suggested that several minds working on a business would generate insight that no one person would see. Multiple organizations can help you get one. Dynamite Circle offers a mastermind as part of its membership. Microconf has a matchmaking service, and there are others.
Set a budget. After applying the above, you probably need to spend more on different things! A trip to be where your customers are. Memberships in communities. Marketing tests. I have found that setting a budget for what I will try to spend on my business is as much about making sure I spend the money as it is about not going over. I am usually more willing to spend my time than my coin. That is backward. I can earn more cash. My time is gone forever.
Finally, remember not to die. "Bet the company" moves will earn you plaudits among the connoisseurs of "hustle." Car crashes win the plaudits of racing fans. Your job is to win the long game, and that starts with survival. Give yourself the room to feel safe and smart.
Good decisions come from a position of strength. And there is no better strength than having the runway, energy, and wisdom to continue the game.