Talk about what you need, not what your partner needs to do. When someone is approached "honestly" about their shortcomings and lacks they are likely (and understandably) going to get defensive and refuse your request or seek revenge later. Gottman recommends a 'softened startup'. Example: I'm feeling overwhelmed lately. Would you help me identify some ways I can get some more support?
Identify topics that are off-limits. Even after you've decided to be honest, some topics are too difficult to tackle, especially if they've been problematic historically in your relationship. Setting some ground rules about what issues shouldn't be talked about at first is probably a good idea. Example: We both get so upset when we talk about money. Do you mind if we wait and talk about this with a counselor?
When necessary, wave the white flag. Sometimes intense discussions are difficult and exhausting. If you're trying to be honest with each other, make sure you allow one another to "drop out" at some point for a cooling off period or some down time. Example: I want to resolve this with you but I need a few minutes to cool off. Can we take a ten-minute break?
Be mature. No name-calling, derisive remarks, or belittling contemptuous comments should be allowed when carrying on an honest conversation with a loved one. Example: I have something important to discuss with you, but it's a very sensitive topic for me, so could we schedule a time to discuss this?
Allow for a few white lies. In every honest relationship there comes a time to keep one's mouth shut. Example: Yes, that skirt looks lovely on you!
Aim for progress, not perfection. Honesty in a relationship requires trust, time and tolerance. Sometimes your spouse shouldn't be the first person to hear about how they're pissing you off, even if you want to tell them. Availing yourself of a confidante in this case is extremely valuable. You can come clean, unedited, with a close trusted friend and then talk to your partner once you've cooled off.
So the next time he asks if he's the best lover you've ever had, or she asks if she looks attractive in a particular dress, remember that honesty is sometimes less important than kindness if you want to be happy together over the long-run.