If possible never deliver a final mix for important clients right after you finished working on it but rather sleep over it. Very often, when you work for a long time on a piece you either start to not hear problems or start to hear problems that are not there or in worst case both and work against that in your mix and when hearing it with fresh ears the next morning, it sounds and feels completely different.
This effect happens very often and can be quite annoying especially in a scoring session when you need to concentrate on small details for many hours where tired ears can become a real problem. That’s why on such critical situations, it is mandatory to not be alone but to bring at least one more person with trained ears to maintain a fairly neutral perspective.
Another good strategy if possible is to make frequent breaks. Some mixing engineers I know have a 15 min break after 45 mins of working. While this seems excessive this of course has nothing to do with lazyness but is essential to keep a neutral view on the music.
However eventually, everybody needs to find his/her own way of what works best.