There are many ways to give constructive feedback to your designer, and there are even more ways to give destructive feedback. We have some tips to make things run smoother.
Take an active interest in the project. Be willing to learn from the designer and make decisions based upon your goals and their advice. If it's clear that you aren't interested in the project, this sends the signal to the designer that they don't need to be either.
Be constructive with your feedback. Simply saying 'I don't like this' is useless feedback to give to a designer. Instead, give reasons why you don't like it and how you would like it to be different. A good designer will welcome constructive feedback.
Just as important as how you give your feedback is how you take your designer's feedback. They want to create something special for you. Having proper discussions face to face or over the phone are far more constructive than sending comments back and forth through email. You should consult with them on the best approach and understand that while you know your business, they know web design.
Have one point of contact on your side to relay feedback to the designer. Few things are more frustrating for a designer than getting multiple conflicting comments from different employees on your side. You need to establish one person whose job will be to collate the feedback and relay the final decision to the designer in a useful way. This person must also have the authority to make final calls about the design. Decisions need to be made throughout the project's journey and not when your CEO has a glance at it the day before launch and doesn't like the buttons.
Take payment seriously. The web designer has provided you with a service and they deserve to be paid for it. Stick religiously to any payment plans you have agreed with the designer and show them the proper respect that you would like to receive. Trying to reduce the costs by suggesting that the project will look good in their portfolio or you'll promise them more work in the future is a terrible idea. Prompt payment will result in a higher quality of work.
Don't be vague. Avoid ambiguous phrases like 'It has to pop' and 'give it more depth' and instead be specific about what you would like changed on the site. You should be telling the designer the problem and it's their job to give you the solution. Phrase your feedback 'we would like more emphasis on the news articles' rather than 'make the news article text bigger'. There is usually a much better solution that your designer can suggest it to you if they know the problem.
Don't give feedback for feedback's sake. It's tempting to feel like you have to say something to your designer, but unless it's constructive then it can cause harm to the design.
Avoid 'design by committee' at all costs. The end result should never (and will never) please everyone. Do not feel you have to implement everybody at your company's feedback. Carefully hand pick 2-3 people to share design iterations with and collect feedback, however make sure that one person has ownership of the project and can make the final call. Do not give your designer a list of every employee's opinion. They need a final direction from you. If you have very conflicting views on your side then call on the designer's expertise as their opinion as an outsider can make them extremely valuable.
Don't get personal. Your relationship with the designer is a professional one and should be treated that way. If things go wrong, nothing can move it further from a resolution than unprofessional acts or comments.