1. The most vital - NEVER agree to discuss anything on the phone. There is more than one reason for this; unless you record it, you will have no proof of what was said, which will be awkward if the caller happens to be, say, an unscrupulous journalist who will twist your words, or an entrepreneur who is good at forgetting promises and denying he ever made them (met both). But my main reason is that it's far harder to say no to a voice in your ear than to an email. Old and wily as I am, I got caught like this fairly recently by a man who insisted on ringing, then badgered me into agreeing to his use of a poem I didn't really want used. If ever anyone says "Oh, I'd much rather talk on the phone, we will understand each other so much better than in writing", remember that what he is really saying is "It will be easier for me to get my own way". Tell him you have hearing loss or a terrible phone line/memory and INSIST on email or snailmail.
2. If it's something the enquirer should be paying you for, mention money very early in the proceedings. Don't just assume they will, because it's quite possible they won't and will hope to get you for nothing. If the mere mention of money sends them scuttling, all the better; at least you haven't wasted your time doing any free work. It took me half a lifetime to forget my mother's belief that it was rude to mention money. It isn't if you want to get your hands on any.
3. If you have a blog and do reviews, folk will ask you to review their books. I don't mind these requests. You see books you might otherwise miss and I enjoy writing reviews anyway. What I don't enjoy is feeling obliged to, and if a small publisher sends me an actual book copy, I feel: well, this has cost them a copy and postage, I really must review it even if I can't think of much to say. My solution to this is to ask enquirers to send an electronic copy instead. I don't mind reading onscreen, and when I know it hasn't cost them anything, I feel better about emailing back, if necessary, "sorry but I don't think I'm the right reviewer for this".
There you are - gratis, the stratagems of a chronically shy person who cannot actually echo Gideon from Local Hero in saying "and are there two Gs in "bugger off?" but who thinks it quite a lot...