Use closed back headphones or ear plugs to reference the low end of your master and set the upper limit of bass levels. This gives you the most direct sound and the best perception of bass frequencies, especially if you are not working in a high end studio.
For checking the mid range, you can use speakers that are limited to a tighter frequency range, like mix cubes, kitchen radios or smartphone speakers. The reduced range makes it easier to spot problems in the middle.
Studio monitors and headphones provide enough high end to adjust the high frequencies of your master. Turn up the volume to spot harsh tones.
Try not to make the perfect master for one device, but a master that works on all devices. That means you will have to make decisions and compromises.
Apply volume-, filter- and pan-automations to point out different parts of a song. Mastering can also be a creative job.
Check your master at different volumes on all devices. It can also help to leave your studio and listen to the master from the next room. (Good to check the overall consistency.)
Give yourself breaks and don't work too long with high volume levels. Listen to your final master after a few days to make final adjustments if you have the option.
Use reference tracks for loudness and compression levels. Turn off Spotify's "normalize feature" to check the real loudness of mainstream productions.
