Email the whois contact.

Use an email address that does not show you have a huge wallet (if you do).

Propose a price. Not too low. Not too high. Get a good estimate from estibot.com (or valuate.com, same engine). The estimates are sometimes off but it's the best thing around, and professional "domainers" use it.

If you don't propose a price, you might look like a tire-kicker, and not get an answer. If you do propose a price that is way below the market (ref. estibot), then you might upset the person.