What techniques do you use to ensure that a mix translates well across various playback systems, from high-end speakers to smartphone earbuds?

One of the most important things is to check on those devices. Export your mix and listen to it on them. 
Knowing how music sounds on those devices guides you to apply the necessary changes on yours. 
I have presets on my Equalizers that replicate the curves of Iphones/AirPods/Small speakers/Car Speakers etc. You can do that too and quickly/easily listen to them. 
Having a full harmonic content and a balanced mix helps your productions translate everywhere. 

I invested a lot of money and time in order to ensure my monitoring situation is as flat as possible. This is a great starting point, but I also use a lot of other listening devices to ensure the audio translates well. I would often listen to music from my living room stereo, in the car, from phone speakers or cheap earbuds just so I can have a broad range of idea of how it’s gonna work

To ensure a mix translates across playback systems, I start by balancing the low end and ensuring there’s NO muddiness or excessive bass, which can be overpowering on small speakers. I regularly check the mix in mono to identify phase issues and make sure critical elements, like vocals and bass, remain clear. After mixing, I test on headphones, small speakers, and even laptop speakers to spot issues with clarity. EQ adjustments to tame any resonant frequencies that jump out on different devices can also help the mix translate well.

I don't think there is a specific technique. The important thing is to listen to the mix on different commercial devices. Personally, I usually work on stereo stages and other stages in mono, this is very important. Getting a good mix in mono, where there are no problems of cancellations between L and R will make us have an excellent mix and go listening in different types of reproducers.

For me there's no technique or precision method. It's a matter of feeling and how you listen your track. But for sure if we go to technical aspects many people think that the Low End and High End are the most important thing... but it's not. For me it's the Mid Range that will tell you if your track works or not. That is why sometimes you can listen a track in your studio or headphones and it sounds amazing but then when you take it to your phone or car your track just sucks. 

The Mids are the answer to get a good translation across various systems. 

EDM & Dance engineer here!

As the question suggests, it's crucial to listen to your mix on a variety of playback systems—high-end speakers, car stereos, smartphones, earbuds, and even laptop speakers. Each system has different frequency responses, so testing on various setups helps you identify if any elements are lacking or overemphasized. I take notes on what sounds good and what doesn't across these systems. Over time, this testing process helps you develop a mix blueprint that consistently works across different environments.

It’s also important to recognize that some mixes are designed for specific environments. For example, a techno track that relies heavily on low-end kicks may sound incredible on a club’s big sound system but won’t translate as well on a smartphone due to its limited ability to reproduce sub-bass frequencies. You need to be realistic about what can and can’t be achieved on smaller systems, so balance your mix accordingly.

It really comes down to practice and understanding the target playback environments. By frequently testing and fine-tuning your mix for different systems, you’ll develop an ear for what translates best across them.

The key is asking "what's important to you?".

While a balanced mix tends to translate 'better' in that nothing will cause obvious audible technical problems like overly hidden vocals on a mono bluetooth speaker or cranked sub frequencies vibrating your car doors, you really can't have your cake and eat it when dealing with something as complex as art and music.

Between acoustical challenges like phase/correlation and overtones, and artistic challenges like capturing the right "vibe" that only a washed out reverb part or cranked 808 can deliver, you're gonna lose the plot really fast if you mix with translation as a primary objective. Make a change to suit the smartphone and you'll lose a vibe. Make a change to suit high-end speakers and you'll lose a vibe.

So.. what's important to you? Only you can answer that question! 

If you must have that double-tracked guitar extremely wide in the mix, then be ok that you're probably gonna lose it when listening on a mono bookshelf speaker.

I personally prefer my mixes to translate the best in the best listening environments and on the best stereo systems, cause that's where I'm being most critical when listening to music anyway.

To make sure a mix sounds good everywhere—whether it’s blasting through fancy speakers or squeaking out of someone’s phone—I like to bounce between different setups. I’ll test it on everything: studio monitors, cheap headphones, a car stereo (my personal favorite - I really know my car sound), even the kitchen Bluetooth speaker. The key is to find that balance where nothing disappears in the mix, no matter what it’s played on. And if it sounds decent on the worst set of earbuds you can find, you're golden!