I'm not yet 38, but if I'm not yet experienced enough to teach these basics, I may never be. All of this advice is based on my past mistakes. Take it with a grain of salt, and apply with prudence. :)
  1. If you can at all avoid it – don't negotiate with scrooges.

    You can recognize a scrooge in that they not only negotiate hard, but also low-ball every offer, and seem to want to nickel and dime you with respect to every term and condition.

    If you can avoid negotiating with these people, avoid it. If you are selling, sometimes they will still buy your stuff on your terms, but most of the time they walk away from the transaction.

    You want them to walk away. If you get their business, you will be a slave to them.

    In most transactions, there can be substantial give and take. A fair price is 50, but a buyer is willing to pay up to 80 – with clenched teeth. At the same time, the seller may be willing to sell for as low as 20 before they really regret it. Scrooges are hell-bent on maximizing all benefit to them at the expense of the other. If you negotiate with them, negotiations will either fail, or you will see them walk away with 85-95% of the benefit, while they toss you the remainder as if they're being generous. If you make a mistake, you may have to work for them at a loss.

    Fine-tune your radar and stay away from these people.

  2. Don't be greedy.

    This is the converse of avoiding scrooges. Don't be a scrooge yourself. If you see the other party is to benefit from the transaction, don't be greedy and think this means you didn't negotiate hard enough. A fair transaction is one where both parties benefit. If you're reluctantly willing to sell for 20, and the buyer is willing to pay at most 80, the fair price is not 79. It's more like 50.

    You should be happy for the other party to get benefit from the transaction; just make sure you're also getting a good benefit yourself. If you want to always gain 100% of all benefit, you're being a scrooge. If you're being a scrooge, no one with any experience will want to deal with you. If you become known for that, your business pursuits will become limited to exploiting people who are not experienced, or who have no other options. May the supernatural entity of your choice be merciful to you if you can get a good night's sleep while you do that.

  3. Don't rush in.

    Suppose a person approaches you out of the blue and offers you a deal. Your first instinct is to be enthused, and you say yes! Then you dwell on it and start to regret that you agreed to the terms prematurely. Maybe you could have negotiated and obtained a better deal. Maybe you mentioned it to people around you, and they rain on your parade saying you're getting taken advantage of. Maybe they're just jealous; or maybe they have a point.

    You want to avoid this situation to begin with. If your first instinct is to say yes, downgrade your enthusiasm a notch, and say yes tentatively. Then think about it carefully and consider if the terms that are being offered to you are fair, or if the other party is low-balling you. Pay attention to the principle Don't be greedy, but do not finalize the deal until you're sure the terms are fair to all involved.

  4. Stick to the terms.

    If you have entered a final agreement, then even if you later reconsider, and come to think the terms are now unfair, you have already accepted. You accepted the responsibility to uphold the terms and stick to them. If the terms are now so unfair that they would cause you substantial harm, and it will be irreparable – then by all means, do what's necessary to extricate yourself from the situation. But if the terms are viable, just not as fair as you wanted – the issues you are having are with your pride. Take this time to learn from your mistake, and contemplate negotiating better at the next opportunity. Don't renege on an agreement just because you're not happy with your negotiation performance. Sacrificing integrity can be very costly too, and reneging does not fix your pride.

  5. Chill out.

    You have waited a long time for an investor or a buyer. Someone is showing interest. This must be it! This person might buy! There's talk of numbers, and you stay awake thinking of all that could change in your life overnight.

    Chill out. Relax. Stop counting chickens before they've hatched. Regardless of how eager you are, most deals that are considered don't actually happen. If it is going to happen, overeagerness can kill it just as well as disinterest. Enthusiasm can make you accept a deal that is unfair to you, if you're only seeing the upside.

    Depending on who you're dealing with, deals can take time. With corporations, simple exchanges can take months, with weeks-long pauses between them. Even when all is agreed, getting a payment may take months.

    Don't bet substantially on any particular negotiation. Plan as if it won't come through. Then if it does – it does!

As you can infer, I have violated all of these principles over time. You can learn and avoid paying the same price. :)