Link building is going to happen whether I like it or not, so I better accept the 949394 emails I get per day. A WebmasterWorld thread asks link builders how they ask for links via email. Some tactics include putting the link up first (and thereby then emailing people -- they often will give the reciprocal link) and keeping the email short and brief and personal.
Ask for the link. Don't demand it.
Offer people you link to something valuable in return. For example, wheel suggests that you say something along the lines of "I've got data your visitors may find interesting" or "Here's a unique coupon code."
Since these are email requests, though, can you be reported to Google for spamming someone else? There's no answer to that -- it depends on the recipient.
I should add from my experience that it's unwise to ask big blogs for links. They might have a high PageRank and is extremely relevant for your purposes, but chances are, old posts won't be updated. (And like I said in sentence #1, we get hundreds of link exchange emails a week. That said, most aren't read anymore.)
Forum discussion continues at WebmasterWorld.