“Asking for a friend” is pretty much universally understood to mean, “I’m asking for myself but too embarassed to admit it.”
So how should you ask a question if, really indeed, in fact, asking for a friend and not yourself?
(Not asking for a friend) 
If it’s not an embarrassing question, why the need to say “Asking for a friend?” Why does the recipient of your question need to know who you are asking for?
How about:
My friend <INSERT NAME HERE> wants to <INSERT EMBARRASSING SUBJECT HERE> ?
putting brackets “Answering for a friend, (really!)” at least makes it clear that you’re not tacitly admitting it’s you. There’s really nothing you can do to fully convince other people though, if you’re genuinely upset the responders will think it’s you.
The specific words “asking for a friend,” I’ve always kind of taken in jest. On the Internet, it’s kind of an idiom on its own.
Earnestly asking something on behalf of a friend – most especially for a serious matter – is typically conveyed by the apparent sincerity of the message (tone, wording, plausibility, etc.). 99 times out of 100, readers won’t give you trouble or try to get you to admit that it’s really about you.
If I am asking for a friend I really am asking for a friend; as a former public school teacher I have no shame at all. If it isn’t someone the person I am asking is close to I will usually name them. If it is someone the person would know well I usually come up with something like “For reasons of national security I can’t say who but someone was wondering what you thought about …”
The easiest thing to do is just not to care what people think.
“Listen, I was talking to a buddy of mine and he was saying that he went down to the DMV and they told him he had an outstanding warrant for a DUI. Guy wanted to know what he should do about it. ‘Heck if I know, turn yourself in?’ I told him, but then I was thinking that you work with DUIs and thought you might have better advice.”
If you’re talking about how you ask on a message board, then what do you care? It’s a bunch of strangers anyway who most of the time we don’t know what we’re talking about, so if someone thinks that your left testicle hangs too low and looks inflamed when in reality, it’s perfectly fine, then what does it matter?
Just ask the question.
Nothing more is needed. No one seriously cares why you’re asking.
Don’t ask the question. Tell your friend this is how I answered the question when I was asked. Do you think I was right?
If the question is along the lines of “Will masturbating into a can of frosting hurt me?”*, then tell him to start his own account and ask. Because there’s no way anyone would believe a friend would ever ask that question of another friend.
*NOT asking, for either a friend or me
What kind of icing and are you using it on a cake for personal consumption or a party? In my experience it isn’t an issue in banana icing but I would avoid the dark chocolate.
Not answering for myself or a friend; just to enjoy the thread.
Honest.