About 80% of the total environmental damage of a smartphone occurs during the manufacturing process, so the single most important thing you can do to mitigate that damage is to hold on to your phone for as long as possible. If it breaks, get it repaired.
Over 60 years of life, if you keep your smartphone for an average of two years, then you'll need 30 devices. If you keep your phone for five years, then you'll need 12 devices. The European Environmental Bureau has stated that extending the lifespan of smartphones and other electronics by just one year would save the EU as much carbon emissions as taking two million cars off the roads annually. The UK Green Alliance has stated that if a phone is kept in use for at least five years, instead of the typical two to three years, the carbon impact per year of use could be cut by 50% and the water impact could be halved.
Once you have decided that you don't want your phone, you must get it to someone who does as quickly as possible. Don't hoard it. Sell it, donate it, give it away. The brands drive aggressive planned obsolescence so the lives of phones are unnecessarily short, and that means that every month counts. If you keep your phone in active use then that's 90 kg of mining waste that's delayed, 14,000 liters of dirty water that's delayed, 60 kg of CO2 that's delayed. So, don't hoard.
Buy repairable and modular. Look for a good warranty. Check out Fairphone, one of the most modular and repairable phones you can buy. Also, consider refurbished or remanufactured. Buy as simple and low tech as possible. So many new features are scams. 8K images. You can't see the difference between a 2K image and an 8K image on a smartphone screen.
Here's some tips for getting the longest life possible out of your battery:
Minimize notifications: They’re bad for your mental health and bad for energy consumption. Disable location services where possible as they consume energy. Delete old content and apps and think hard before downloading new apps. Watch out for greedy apps such as Facebook or Google Maps, that use substantial quantities of energy.
Don't drain your battery completely. Try and keep it between 20% and 80% if possible. Short charges are better. Don't leave it plugged in at full charge. It stresses the battery. Use quality wire to charge. Wireless charging consumes about 50% more energy. Plug out charger when not in use.
Use Wi-Fi to transfer data. Dim the screen as much as you can, and use dark mode where possible. Stream in standard definition video, and remember video already represents over 80% of total Internet traffic. It's a huge energy consumer and waste creator.
But the biggest tip of all: Hold on to that phone!