Learning how to ask and receive feedback is an important skill for a designer. Providing useful and constructive feedback is an important skill for a client. Feedback is very important in the completion of a design so that it accomplishes the goals of the project. Often, feedback from clients is vague, "I just don't like it..." or approaches the issue from the wrong angle, "Make it bigger!" instead of perhaps the more useful "Add emphasis here."
During a design project, I tend to ask for feedback at different intervals. For example, if I'm designing a logo, I'll ask for feedback after completing the first round of drafts. Then I take that feedback and further refine several of the designs. Then I get more feedback on those designs to finally create the finished logo. I have a similar process when it comes to designing websites, mobile, apps, or print pieces (like brochures, postcards, etc.) It is easier and can keep a project on track to ask for feedback early and often. It's better to make changes in the draft stages than in the final product. It saves the client money and keeps me, the designer, happy.
When I ask for feedback on a design, I try to specify the following:
I tell the client exactly what I'm looking for from their feedback. I describe the problem and how the design addresses it so I get specific feedback rather than a "I don't like it..."
I need to understand the point of view of my clients. I will often ask questions when I solicit feedback to make sure that we're on the same page.
If I want to receive useful and thorough feedback, I need to give the client enough time to review the work and to write it all out. Often feedback is developed after several passes. There are the initial impressions when a client receives the files and then there's the more comprehensive assessment on further study.
Giving design feedback in a way that is constructive and supportive is not easy. Be empathic and keep in mind that a significant amount of effort has been invested and most likely you are seeing far from the first version. You should be direct enough, but not too harsh, and never make it personal.
Saying "I just don't like it" or "Just make the logo bigger" isn't helpful. Think of what problem this particular design is trying to solve and evaluate the design in relation to that problem. Instead of "make the logo bigger" perhaps suggest that you want your brand to have more emphasis. A designer can do a lot of things other than making the logo bigger for your branding have an impact.
Try to see the big picture. Often there are many technical restrictions, for example, designing around space required by the post office on postcards and mailers or the limitations of the software being used to build a website. There can also be requirements based on the target audience, budget, etc. that will constrain what the designer is doing.
Asking your designer questions leads to a better understanding on both ends. If you're wondering why the designer designed something the way they designed it, ask why they did it that way. This way you get context and reasoning behind the designer's choices.