* Build what you're interested in, and use the product yourself. If you're not personally using the product, the loneliness of a solo op can quickly translate into "why am I doing this at all?"
* Don't be afraid to admit what you don't know, AND don't be afraid to spend extra hiring professionals (or freelancers) to do those things. Consultants work the best, in my experience: hire them to build out some kind of framework that lets you do what you need to do without them, and then bring them back as needed to do more advanced things you can't do.
* Automate everything you can. People say startups should do things that don't scale, and that works when you've got the resources to throw at things. However, if it's just you then time is often your most valuable resource (moreso than money, in many cases): automate everything you can that takes up more than 30-60 minutes of your day so you can focus on more important things. This also ties into point 2: automate out the drone work so you also stay interested in what you're working in and don't succumb to a brick wall of "why am I even doing this?".
* As a solo founder, you need to be self-driven and self-accountable. You need to keep working and Get Shit Done even when nobody is asking you whether something's done or not. Don't be afraid to take breaks or time off when you need to recharge, but don't fall into a habit of "nobody's asking for this; I'll get to it later".
* Lastly, it can be tempting to vastly expand your work hours (especially if you're feeling pressured to get something done or feeling overwhelmed), probably more so than if you had a cofounder even (IMO it's a lot easier to suffer through long hours yourself than to command someone else do so also). There's nothing wrong with a 60 hour workweek, but you also should never feel like it's necessary. Sometimes I cut myself off at 8 hours in a day even if I'm on a roll and feeling motivated; that motivation often builds up and translates into another strong day tomorrow. You might also get a ton done in a single day by working into the night, but it's easy to translate that into sleeping in tomorrow because you got so much done and having a much slower day in general. Find what works best for you, and don't let yourself get burned out.