Negotiating both terms and price is absolutely essential. How long it takes you to develop the necessary skill depends on a) where you started, b) how much you "practice" and c) your personality type.
It has taken me, as a Real Estate Professional, a great deal of time to develop the negotiating muscles necessary to confidently assure my buyer clients that we have the best deal possible.
Someone who is purchasing a single home to live in every 5-10 years obviously doesn't have the experience to negotiate the deal for themselves, so they hire me and my commission is paid out of the proceeds of the sale. Trust me when I say that the amount of money, time and anguish I save my clients is totally worth it.
That said, as an investor, you may not want to hire a professional negotiator as it's a skill you absolutely want to develop for yourself.
First step: Completely qualify the sellers mindset with regards to 'need' to sell vs, 'interested' in selling. If they are "interested" walk away.. as an investor you simply will not talk them down to a price worth paying. You could use the opportunity to practice your negotiating skills in full knowledge that you are NOT going to buy the home. Personally, I'd save my energy for one that might result in a purchase.
Second step: Stay in conversation a lot of probing questions that tell you everything about the house, the work they've done on it (and not done on it), what they love about the house, what the hate about the location and so on.. drill down and find multiple ways of asking the same question in case your not getting clear responses. They may begin to get annoyed so you make sure to express interest in solving their problem.. (buying their house). Don't talk price right away.. just keep asking questions, expressing interest if the right terms and price can be negotiated.
Third key step: Do NOT become emotionally involved with the outcome.
Fourth: By now you should know alot about the house and the seller as you've developed a feel for the situation as you talked. Use that knowledge. Did you learn that they have a new job and need to get this off their plate ASAP? Great, ask them "If we can agree on price and terms, are you ready to close tomorrow?" Note that you are not asking them to close tomorrow, your asking if the COULD close tomorrow so don't panic if they say yes.
NOW you have a good idea of motivation. Thank them very much and let them know you will be back in touch to make an offer.
In my opinion, negotiation is almost over at this point. You discover what the house is worth to you and offer that, not a penny more. You try to make all other terms so appealing to the seller that they say Yes. If they don't, you thank them for their time and wish them luck.
Call back in a week.. and offer it again.
The seller spent the time between the qualifying conversation and the offer thinking how great it would be if you bought the house so the whole situation would be done with. The offer was so low they said no but a week goes by and nothing happens... the thrill of possible sale replaced by the drudgery of trying to sell.
Don't forget to represent yourself as the person who can solve the sellers problem. All other buyers will pale in comparison as they try to nickle and dime the seller. You aren't doing that. Your telling the seller what you can afford in order to solve his problem.