A fabulous and well known Designer that I once worked with when I was just starting out, shared a great tip. It was one of his tricks. I since have noticed it to be true, and incorporated it with great success.
He said: "The trick to designing film & TV is to pick a theme, and carry it through".
In film and in the real world. It's true for any design, whether its Architectural, Interior or Industrial. So we often create Mood Boards.
Thats logical and straight forward, for a set build, as in Architecture; the mouldings carry throughout as a family, the colour palette remains consistent. It is a really important aspect of Production Design, especially in film, when scenes are shot in diferent geographical locations, because it prevents the audience from getting lost.
For a location / set / location match, it becomes crucial to notice the design elements that can tie the different locations to one, as is being depicted in the story. Sometimes the PD will see it, and direct the Set Designer, however not always; this is sometimes an opportunity for the Set Designer to excel. Often details like tie-in designs are left to the Set Designer's suggestions. Often this is one of those "small details" that when executed either well (never noticed) or poorly: can become a "big issue". To tie a scenic plug into a location, if there isn't a common feature to carry through and it needs one, sometimes its a good idea to create one, eg: conduit, greens... As a Set Designer pitching creative ideas, always have a photo reference to prove your design to be credible. It means you did your research. You diddn't just make it up, even if you did. It looks good and it builds their confidence in you.
For the successful overall look and style, the film needs a clear design, tasteful and appropriate to the story. The Characters also need design. Their wardrobe tastes, their furnishings, their car; all sells their personna to the audience. Consistency is key. Do they shop at IKEA? Sometimes they do! But I know people who also wouldn't be caught dead in IKEA.
And its been a really helpful tool for brain drain. Sometimes I have relied on the design and mood boards heavily for inspiration when I have felt over tired, and wandering off course...
Mood boards really help the production, however they often end up in the SetDec office (no offence). Best practice I've seen in action is to digitally photograph them and send them out as a PDF photopackage with the Tech Pack.