Stereo vs Mono Mix / Phase Questions – Production Techniques Forum

I have been reading that it’s important to see if your mix sounds good in mono to make sure it will translate to different systems. Lo and behold when I hit the mono button on the master bus of a recent project, a bunch of information seems to disappear. I understand this has something to do with phase cancellations but how do you fix this? What can you do in the creation process to prevent this from happening in the first place?