Time and Value
I have recently received a few unsolicited email requests from people whom I don't know. Now, I am actually happy to help other people, but I have to weigh this desire to be helpful with the actual value it will create in the universe as a whole, not simply to unknown individuals requesting something. Time is inelastic and I have a limited amount. The older I become the more I desire to help others and the more picky I become in providing my time.
Priorities and Protocols
Priorities are going to be something along the lines of how much value is created for others, not just the individual doing the asking. In addition, the way that the request is made does have some value, since it is after all a favor that is being asked and a little bit of gratitude goes a long way (while fawning flattery is a sure sign of someone who just doesn't get how to ask for things and what it means to be appropriately grateful). That said, there are a few suggestions which can help provide both:
Introduce yourself. This sounds basic, but it is amazing how I get emails from people I don't know (or don't remember I know) who don't actually clarify how we know each other or how the individual found me.
Part of the introduction is about how we have something in common. This can be anything from where we went to school, to how we share similar interests. If you found me via my blog or other websites, or friend-of-a-friend on Facebook or Linked-In, then surely this should be stated. Even the path via Google or Twitter can be interesting. I have made good friends and found vendors and clients this way. One part of connections is a referral. However, this can be a two-edged sword (or rather the proper metaphor, a sword that cuts both ways). For example, if someone whom I have had bad experiences with is making the recommendation, then there is actually a negative connotation. This recently happened and while it is unlikely the individual innocently making a request knew of previous less-than-pleasant experiences with the recommending party, it should be kept in mind and a question about "how they know you" should be made previous to using that name in an attempt at contact. When in doubt, simply don't use the recommending person's name.
This is the heart of the matter. Can you ask specifically for something? Generic advice, while a somewhat flattering request, in my experience is usually the most ignored kind of advice. Why should someone waste a lot of time trying to comb through their memory assembling something along these lines? Ridiculous, actually. But this kind of question is asked all the time. The art of asking has a lot to do with what constitutes a good question. Ask thoughtful questions and it is much more likely that one will receive thoughtful answers.
Result of Assistance
This is the potential result of assistance, namely value. But again, value to an individual ("I need this... so I can...") is much less interesting to me (if I don't know you), than the potential greater result of how others will benefit. Again, be specific, not general. Gratitude should not be the result of assistance "I will be grateful if you..."--that is a result of doing a favor in general--but some concrete potential outcome.
If you need something from me that I am not obligated to provide, you don't know me, and the only value will be to you, then don't ask. This is the attitude of a person who simply wants others to help them out with no real thought. If time were infinite, then this kind of a request might make sense. But it is not, and the time available needs to be put to use helping those for whom the assistance is more urgent and has real value beyond the individuals themselves, namely those helping others.
Now that said, of course the small courtesies of in-person interaction such as "can you help direct me to the nearest gas station," etc. do not apply here. That is simply being neighborly. The same would apply to social networking and the like, such as Twitter, etc. After all, asking a question can be a way of making friends, I understand all of this. It is the random request, without introduction or a clearly stated objective, one which is usually a part of a broadcast of emails to many such individuals which is simply inappropriate.