First of all, never ask a realtor for advice. The realtor represents the SELLER.
Blankip's advice above is by far the most accurate of the previous answers.
The first step is to estimate the market. Look at past sales in the neighborhood over time, and from them estimate the prospects for the house at different time durations. Based on other sales, how fast do you think the house will sell at a given price? 60 days, 90 days, a year? If a house is high priced, that means the seller is prepared to wait. He is saying "I am happy to wait a year to find somebody who will pay this."
Next, who is the owner? Young professional? Retiring couple? Landlord? Flipper? Who is it? The more you know about the owner, the better. Everybody has a time table, you need to find out what that is.
Next, what is YOUR timetable? You need the house by the end of the month, or by the end of the year, or never, which is it?
Objectively rate the house. Plusses and minuses. Good houses are those which everybody else hates and you love. You will get the best price there.
(Assuming you need to find a house in 90 days) Based on these considerations determine the lowest price you think the owner will accept in a 30-day time frame. Make a written offer with an address and email, no phone number. If he comes back with a counter offer, ignore it. If for some reason a realtor has your number and calls you, tell them "My written offer speaks for itself. I have nothing further to say."
It is very important not to entertain haggling or counter offers. Don't even pick up the phone. He has your WRITTEN offer. He can email or write you: I accept.
If the 30-days elapse, move onto your #2 choice and make a more aggressive offer. If that doesn't work, go to choice #3 and accept the listed price.
This strategy may seem counter-intuitive because the natural tendency for people is to want to communicate. Trust me: the way to succeed in a negotiation is to NOT communicate. Make your offer and that is that. That is the pro way to do it, and will produce the best result for a short-term situtation.
Long term situation
If you are an investor ("flipper"), or have a lot of time to wait/spend, you can use a different strategy which involves pressuring the seller. What you do here is find a property you want in which the owner is vulnerable. That means someone who is old, bankrupt, out of work, indicted and on their way to prison or already in prison, etc. Bank owned properties fall into this category.
In this case you figure out the 6-month price or however long you are willing to work on it. Then you pester the person. Become their buddy. Visit them in prison. Take the bank officer to lunch. Show up on holidays. Invite them to Thanksgiving. Start a relationship. Every two weeks you pester them. Want to sell yet? Want to sell yet? You basically harass them until they capitulate. Maybe it takes 6 months. Maybe it takes 2 years. Eventually they will give in. By this means you can get a much better deal than in strategy 1 above, but it takes a lot more time and effort and is appropriate more for an investor.