If I'm speaking to a doctor, counselor, or other professional, I would be seeking professional advice. And even if I just wanted someone to talk to, I would understand if the person gave their expert advice. But I'm to talking to family, friends, and other people who aren't professional advice-givers, what makes me trust someone is hesitation: Questioning further to make sure they understand. Offering advice as a question, uncertain that the suggestion is right.
I've talked with people who "knew" what was right for me and people who were more hesitant. I've found that everyone I can trust, everyone who really means it when they say "Do what's right for you," has at least a trace of hesitance. It's a trait that I'm actually trying to exhibit more when talking to friends about problems.
When discussing a friend's problem, hesitance isn't about a lack of confidence - it lets your friend know that they are in control. A common root of most problems is the lack of control over your own life, so telling someone what to do can make them feel worse. When you question your friend further, you're making sure you understand the problem and letting your friend know you're taking them seriously. Offering advice as a question such as "Have you thought about..." shows that you understand how serious or complicated the issue is and that there may not be a solution that fixes everything. It also shows that you aren't judging your friend, and that if that suggestion doesn't work, you'll keep coming up with ideas. It lets your friend know that you want to help but you are not going to impose anything on them. You're giving them back the control that they may have lost.
I have heard lots of people say, "Do what's right for you," and then not accept people when they actually do it. If you are going say this, make sure that you really will support the person in taking your advice.