I'm becoming increasingly confused about the correct way to form questions in informal speech. I originally learned that I should replace か with の. Then I learned that that was a bit girly. Then I learned that I didn't need a particle at the end at all, I should just raise the intonation. Another book told me I should be using かい and だい. And finally, it seems I can use か after all. What is the truth here? What do everyday Japanese people do (let's stick with Tokyo dialect)? Just to confuse things further, I'm familiar with the concept of the explanatory の, but how does that interact with all these schemes?
の sounds a little feminine but you can use it if you're male - so long as のか could also be used (兄弟いるの？ you can say but not どういう事ですの？) It also has somewhat of an implication of asking for an explanation sometimes (何やってるの？ - What are you doing [and why]?)
Asking without a particle is probably most common informally - 手伝う？ It has no real hidden implications, so feel free to use it.
かい and だい are largely used by male speakers very informally, with people their own age or younger. They sound soft. かい is used for yes/no questions and だい for open ended questions - remember that だい has an origin related to だ, so you can only use it where you could use だ. Saying どうだい？ is fine, as is どこへ行くんだい？, but not 何してるだい？ Another note is that かい can very occasionally be used for non-yes/no questions. Anyway, these two are less popular among the younger generation.
か sounds somewhat agressive in plain speech - それが名前か？ It would probably be best not to use it unless you wanted to make your anger clear. However, following a volitional, it is far less agressive (and often pronounced っか with the volitional おう shortening to お). 行こうか？ It's also masculine speech in plain form besides following volitionals. The last note is that, in writing, where the plain form is usual, か is the usual way to ask questions, and usually followed by a full stop - なぜ行ったか。