Doing your daily ARKS mission on multiple lv50+ characters yields 1 silver key each and everyday - and the exp from them isn't that bad either, plus you can turn them in on a class of your choosing, doesn't have to be the one you completed the daily on. Then for completing 15 daily ARKS missions, you get 1 gold key per week on each character.
If you just have 3 free characters, that's 21 silver keys and 3 gold keys per week. Some people did the math, not counting PSO2 day exp bonus, each gold key gives the exp of 3.94 silver keys so it is better for exp running silver keys individually rather than turning 5 in for a gold key (unless you are doing it for PSO2 day). This expands greatly if you have more characters, I know someone that has 20 characters and run most of them for dailies - that's 140 silver keys and 20 gold keys a week.
I typically save up many keys - then pop AT LEAST 50% triboost and 75% exp boost (make sure you also have 50% from daily orders) and run them all at once. If you are in an alliance with a good EXP buff from the photon tree, you can consider popping that too.