Tips for asking for help or advice via email?

I'm a PR newbie. I used to ask a PR expert for help me explaining/clarifying something, few times. Whenever I write, it takes time a lot because I couldn't help writing so long but I think it shouldn't be that long, so I rewrite it few times over and over again. So today, I'd like to ask you for some tips when composing an email to an expert. If you are an expert, what do you want it would be?