Here are a few approaches to feedback to increase the chance that the feedback will be received in a positive light and therefore be effective in bringing about positive change.
First, make sure to give positive feedback when it's due, and be sincere. Rather than saying something generic, like “You're doing a great job,” say something specific about the great job they've been doing, such as “I noticed the attention to detail you put into your latest report. I really get the sense that you were thorough in your research.” Your feedback, above all else, should be genuine. By taking notice of their efforts, they will know that they're appreciated, and will be much more susceptible to constructive feedback when the need for it presents itself.
Address concerns head-on, as they appear. Avoiding the conversation in order to spare someone's feelings might seem like the more polite option at the time, but in actuality, you are robbing them of the opportunity to improve their work. Also, by sweeping small issues under the carpet, you're setting the stage for a bigger, more serious mishap down the line. By approaching it right away in a straight-forward manner, you are empowering them with the ability to address the problem before it spirals out of control, while at the same time letting them know that you respect them as your peer to correct the error on their own.
Always present the problem in tandem with a solution. This way, even if the feedback might involve pointing out something negative, you are giving them an outlet through which they can redirect their efforts in a positive way.