DISCLAIMER: This approach only works with men that have already demonstrated some degree of a sincere interest in you - guys that have shown good will towards you by taking you out, sharing a real conversation, being genuinely affectionate, etc. A guy that you had a one night stand with does not count. A guy that flirts with you all the time, was texting with you a bunch but then stopped and never took you out does not count. A guy that you only meet up with in bars or clubs does not count. You can only expect an honest answer from a guy that has already demonstrated honesty towards you in some form or another.
Wait long enough to make sure he isn't going to pursue you anymore. This will vary depending on the relationship, so you will have to gauge it yourself. There isn't a magic number of days or weeks after you hear from him last. In fact, you may even still be in touch with him regularly if you haven't made him initiate contact or haven't cut him off yet. You just need to be honest with yourself. This is most easily done by thinking: "if he was acting towards my friend the way he is acting towards me now, would I think he'd stopped pursuing her?" If the answer is yes, you're good to proceed.
Contact him. Yes, this is technically "initiating contact," but it is OK when you have already given up on the idea of a relationship with him (so make sure you really have - go back and repeat the step above if needed). The best methods of contacting him are as follows:
- Text - This avoids awkwardness and allows you to lead him into the conversation gradually. See below. Also gives him time to think about what he will say.
- Internet Chat - Basically the same as texting. Good for the same reasons.
- E-mail - Use e-mail only if he is a conscientious/polite guy and you know he will answer, since e-mails are very easily ignored. If he will answer, e-mail has the advantage that it allows him time to really reflect.
- Phone - Not a great option since he is likely to avoid your call or give you hasty answers. Not recommended.
- In-Person - It is almost certain that you won't be able to get him to invest the time it would take for this (though it is a little more likely after long-term relationships). It is also awkward for both parties and forces rushed and therefore inaccurate responses. Not recommended.
Lead him into the conversation. Although guys will ultimately be willing to give you advice if you really want it, they will be extremely hesitant until you assure them that it is important to you and that you can handle whatever you tell them, no matter how harsh. Otherwise they will be worried about offending you. If it's been a while since you've been in touch, you will also need to make sure he realizes that this isn't an attempt to get another date or re-initiate contact for relationship purposes. So you need to start with a message that (a) gets his attention and (b) communicates your openness. Prefacing a question by asking permission always indicates gravity and raises intrigue. So I see the conversation going something like this:
HIM: [probably no response, but if he gives you a negative answer, continue anyway with the following...]
YOU: "I need someone to be really honest with me. It's probably about time I heard it. You seem like a safe learning experience."
YOU: "And I swear I can take it. I only want the complete truth."
[Then you should throw out at least one example of something he would be unwilling to tell you for fear of crushing your ego. This will make him more comfortable with being honest, though it assumes that you are ready to accept whatever he throws out there.]
YOU: "Do I need to work out more? Maybe lighten up a little bit?"
HIM: [At this point he should give you some kind of response. Probably it will be positive. If you get nothing or a negative answer, be persistent: reiterate your need for the advice and that you can handle whatever he tells you.]
Don't belabor any single point. You really just need an overview, so don't try to dig for too much detail. Once you get the general idea of what he didn't like, move on. For example, if he says you weren't dressed well or wore too much makeup, don't ask what look he would have preferred or what would have been the perfect amount.
Don't object to anything. By asking for his unabashed advice, you are in no position to argue. And really, you shouldn't want to - you are merely collecting facts about his opinion. You can process them later. And while I would dissuade you from dismissing any of them, it won't get you anywhere to convince him that they aren't true.
Push past Mr. Nice Guy. If he starts giving you the typical bullshit about "we just didn't click" or "I didn't feel chemistry" it is only because he is not convinced you can handle an honest answer. So respond to those comments with reassurance that you can handle it, and tell him that you need concrete responses. Try this:
HIM: "I don't know, I just didn't feel it."
YOU: "Mike, I am not saying my feelings won't be hurt, but I need to hear the truth so that I can improve. I need to know the concrete things that were off. There must have been something."
[and if that doesn't work]
"Even if a lack of chemistry was the underlying reason, can you tell me some other way I could improve? I know I am not perfect." [Then throw in a few "tough" examples like you did at the outset, and assume that his answer is the real truth, not "chemistry" - because it is. Chemistry is just the cumulative effect of many small things; it isn't magic.]
Give him time to think about it. If he is still hesitating, and you've tried reassuring him that you can handle it, ask if he'd like some time to think about it. Be persistent about following up. Ask him if he needs "a few days," and then get in touch again in a few days.