I have been playing with the assistants a lot and mostly found that the longer and more detailed my instructions are the better the assistant does what I want it to do. It can be a little frustrating at times but most of my assistants are at least a full page prompt, some more. Some of the prompts include specific examples.
The thing I have struggled with most with to get consistent is output in JSON. I ended up having a separate paragraph at the end of the prompt, like a mini chapter titles 'Output formatting instructions" that tells the assistant exactly that it needs to, every time output ONLY a JSON object and with what attributes.
What is really great about the Assistants is that you now have 32k space for JUST your prompt, so you really can put in an enormous prompt, including a lot of ‘shots’. Organizing them properly will take a bit of trial and error.