I'll second KQ800's, and others comments, especially the part about doing your homework first.
For example, fellow wanted my help writing a grant proposal for a business he wanted to start, but after just a little bit of chatting, it was clear that he didn't even have a business plan. Only an idea. And he was trying to get a grant based upon an idea, not a plan. I wouldn't have minded chatting with him more about the grant proposal, but instead steered the conversation in the direction he really needed to start, which was nailing down the business plan. If I helped him at all, perhaps I helped him see the many things that he did not yet know, and where to find those answers.
In still other areas I've been asked for my input, and as long as the one asking is at least moderately prepared, the experience is much more enjoyable because the conversation is then 2-way as opposed to 1-way (as in the first example).