かな is used by males, it sounds weird if females use it. Thus females use かしら。
Girls often say かな, but a guy would never say かしら as that sounds very feminine.
Kana is usable by Japanese women, but mainly for monologues - it's more versatile for usage by men. And yeah, go with the plain form if you're under 40.
I was told by a japanese friend that using かな with the polite form (such as 〜ですかな...) really sounded "Ojisan" (litterally, "speaking like your uncle", which does not sound young and trendy). Maybe this is the reason why it has been written in this article that 〜かな is male speech?
I'm not sure if its a usual thing, but in Tokyo a lot of my female friends often use ...かな〜. It could be a regional thing or that I have particularly unrefined friends, but I thought I'd drop the thought in anyway.
How would you express a desire for something to not come true? such as, "I hope the weather does not clear up." ?
かな is made up of か, a particle for interrogatory and な a particle to express one's emotion. It is used to ask oneself a question or a question to another as to the degree of uncertainty involved. When you use it in the form of ないかな, you are expressing a desire for something to come true. 間違えたかな = Did I screw it up? 天気にならないかな = Hope the weather clears up!
Three main uses of かな. 1. A question with little doubt. e.g. ジムならうまくいくかな。As it's Jim I wonder whether it will go well. 2. Talking to yourself. ゲーセンに行こうかな。Shall I go to the game center? 3. Expressing wish / hope. 誰かこないかな。Is anybody going to come? (I hope somebody comes.)