Your mixes will sound different on each format and on each set of speakers. You are looking for the happy medium called "translatability." Your mixes should translate well to other systems, but they will not sound the same. An iPhone speaker cannot reproduce the same frequencies as your monitors, listening back in mono is different than stereo, etc., etc.
Two things that might help. 1) Make sure you are using reference tracks. Whether files or CD's, have music available-- that you know well on your speakers --that you can reference or "A-B" during mix down. This gives you a standard to shoot for. Make sure you volume match (the reference tracks will probably sound louder, and therefore "better" to you, so you'll need to compensate). 2) Even if you have great monitors, you should have some additional speakers or listening set-ups to check potential mixes on. Car stereo, boombox, headphones, Bluetooth iPhone speaker, etc. can help you make tweaks to find that happy medium.
If all else fails, send a track to someone else to mix and/or master, and if you like it better, ask them what they did.
GJ