The most helpful critiques I've been in start with the designer specifying what they want feedback on. If you know you don't have the visual design in a great place, but you want feedback on a specific interaction or flow, make sure you mention that. The actual fidelity of what you present depends on what you are sharing. If you want feedback on a layout or visuals, showing high fidelity mockups is helpful. If you want critique on an interaction, showing a prototype is best.
1. Don't be defensive.
I know this is difficult, we've all been there. Try not to be defensive, it's awkward for you and everyone else.
2. Take notes!
It's easy to forget what people said, take notes so you can try to remember as much as possible. Sometimes it's that random little comment that will spark an idea.
3. Gather the work you want feedback on in one place.
Hopping around from document to document can be distracting, people don't want to see you flipping between a million open tabs and folders. Be prepared.
4. Don't listen to everything all the time.
Remember, you don't have to follow every little suggestion every person makes.