Adding the Category
There's a really good cheat for communicating synecdoche:
No one gets in. White, black or Cherokee Indian. (Chandler, The Simple Art of Murder, page 24).
Chandler used synecdoche -- Cherokee Indian stands for every race. So he got the vivid imagery that he wanted. But he also mentioned the category (no one), making it clear what he was talking about. Metaphorically speaking, he was having his cake and eating it too.
That technique always works. Duh.
If all else fails, mention the category.
Solving my pretzel problem:
I walk around the room in my bare feet, looking for any food that might have been left on a table or on the floor, like a broken piece of pretzel.
That cut the Gordian knot! How simple! How easy! How brutally effective!
And cowardly – I've took the edge off of my vividness.
The advice here is exactly the same as for any figurative expression. Trust in your synecdoche and use the strongest version, if you can. But when the strong version doesn't communicate well enough, back down. So, if you have done your best to make the synecdoche work, yet it still doesn't, add the category.
They [news reporters] were just diligently doing their job, covering mundane events – funerals and fires and fairs. (The One & Only, Giffin, page 164).
Is That Really Synecdoche?
If the category is mentioned, is that really synecdoche?
No – once you mention the category, you don't have synecdoche. But . . . synecdoche plus category is different from category plus examples. And understanding that subtle difference is a part of good writing. (And good writing is what I care about.)
Synecdoche Versus Examples