Try and find contact details for the domain owner on the site itself, whois, domain auction sites like sedo, or just googling the domain name.
Send an email inquiring about whether the owner would be interested in selling or not, and what sort of figure they would interested in. Don't put an offer in your initial email, try get them to make the first move.
If they respond with a price, use it to gauge how clued up they are. If it's below your valuation of the domain, go for it. If it's above your valuation, negotiate. Start with a value dramatically lower than what they're asking.
A lot of the time the first price is purely speculative - I've picked up domains for less than half the original asking price on the first offer. Try and close the deal but be patient, wait up to a week without receiving a reply email before contacting the owner again.
Sometimes you might find the domain owner is a complete newbie speculator - someone who doesn't know much about the internet and the online economy and has bought some random domain and completely over valued it. If this is the case and you hit a brick wall, unable to convince them their domain is more likely worth $$ instead of $$$$$ (prdeal.com anyone??), there's a sneaky trick that can work. Buy a similar domain (for example prdeals.com which is still available...), and reduce your offer but add this domain to it, suggesting that they get to keep a domain just as valuable as the one they're selling.
Another good tip is if you want to be really aggressive slap a time limit on your offer - this can work well with the right type of prospect, but can immediately turn off others so use your judgement here.
Good luck.