We all know that it's important to test your mix on several systems while mixing a track. What I'm going to suggest however, is you start doing this much prior to the mixing stage. In fact, I have found it really helps to listen across several systems when actually designing the sounds and synths.
The reason I suggest this is because mixing is hard enough sometimes, by taking some of the time while actually designing the sound to make it fit better, you can save time when mixing.
It is always good practice to use sounds that mix well together naturally and arrange instruments which complement the mixing process - a more natural mix sounds much better.
So when you are working on a track, frequently flip between speakers and headphones. I also add an EQ to the master bus, removing much of the bass frequencies and altering the EQ curve to match what a phone or other bad speaker will play.
That way you can flick between the different physical and virtual reference mediums.