Any tips for staying focused on side projects?

Aim to get your products into the hands of actual users as soon as possible, even in a half baked state. Their feedback / feature requests / bug reports will motivate you to keep pushing the project forward.

In general, if you create something, you need an audience to stay motivated.

Sideprojects get abandoned due to analysis paralysis or choice-dilemma forks.

The best way to get them done is to build a frankenstein version of them in 2-3 hours, something that seriously bad, super bad code, bad design, bad architecture, crashes every 2 seconds, hardcoded things everywhere, unextendable, hard to maintain, but even then something that just boots as a vague identity of your idea.

Then focus on fixing it into better iterations and don't forget to go wild on it, instead of sqlite try adding cockroachdb, instead of basic hashmap put a redis there, whatever tech you wanted to learn recently, use it. Then let it be a hulk for a bit and invite some friends to use it. If you are still motivated, focus on best practices, maintainability, remove the overengineering and replace with reasonable changes.

Goal of side projects is to learn something new and have fun. If the fun stops, don't push it or feel bad, just walk away with the knowledge you gained. :)

Work on it in the two hours you have before work, every workday.

My latest project is effectively impossible to complete (SaaS, evolving customer needs, etc), but I've been able to work on it consistently for the last 18 months AND get paid for it.

Before that, I wrote books/articles about React, and before that I learned a lot by building products no one wanted.

In short, consistent effort over a long time helps you accomplish things you think are impossible.

Here's some really bad advice, but it's what works for me (for hobby projects):

Give up and start a metahobby of collecting unfinished projects. The real treasures are the friends you made ^h^h^h^h^h things you learned along the way.

I have found that I learn something and increase my skillset with every unfinished project. That leads to, over the long run, being able to get more accomplished in less time on newer projects. Plus, as a hardware hacker, I now have a tremendous inventory of parts that can be culled from the corpses of previous "failures" with no regret because the project as a whole doesn't function anyway. I've managed to ride out the RPi apocalypse on just my own inventory.

Occasionally I finish something because the planets align, I have all the parts I need on hand (or they're available REALLY quickly), my attention/hyperfocus lasts long enough, and I can reduce scope to a manageable level.

Maybe I'm lying to myself and I'm actually just lazy and don't have enough determination to finish under my own motivation. Meh, whatever, I'm comfortable with it and having a blast.

If you're trying to make money from your side projects, I have nothing for you, sorry. I like my day job and don't need the money enough to let my hobbies become another. YMMV.

Hire someone. Really. Spending a bit of money and having someone who depends on you for work can be a great motivator.

I keep really fine-grained and extensive todo lists. I helps staying motivated if you can just pick something small, do it in 10-30mins, and while checking your todo item, you get the satisfying feeling that your project just got a tiny bit better/a tiny step closer to completion. This works for me even on days in which I don't have much energy or time to spare.

Also, try to not let your project rest for too long, even if you just do the above-mentioned baby steps. I found that the longer I am out of it, the harder it is to bring back the motivation.

You need to understand the underlying issue of why you're not completing them. Too busy? Procrastinator? Etc.

Other answers are methods that may have worked for them (and likely didn't even work for them). You can't apply all those tips, so what really is most relevant to your issue?

I'd check out "The Now Habit" to understand some patterns of why you might be getting demotivated.

Work on it every day even for just a few minutes

Stream the construction of your project on twitch, discord, or make YouTube videos. You would be amazed how well it keeps you going. Also write about the project in a journal of some kind

Don't start another. Just keep working on one.

What kind of projects was it? And do you know why they were hard to complete?

First, don't work on 3 or 2 projects, work only on one, when you finish it you can start another. Try sharing your progress on social media or where ever you want, make a post where you say something like that "I'm going to make an app called X that will do Y and Z, I will start tomorrow, I'm aiming to finish it in 02 months and I will regularly share progress". By just knowing that you have to share progress and that there are real humans waiting to know what you have done, will push you to work hard and don't stop.