Here are some persuasion/negotiation tips from the wonderful ChangingMinds.org:

Avoid floppy language if you want to persuade people: http://changingminds.org/techniques/language/modifying_meaning/floppy_language.htm

Communicate assertively, so that you respect yourself and others. Learn more about this here: http://changingminds.org/techniques/assertiveness/assertiveness.htm

Give criticism in a specific and constructive manner (criticise the behaviour, not the person. Ask what they can do to change the situation / avoid future mistakes): http://changingminds.org/techniques/assertiveness/giving_criticism.htm

Receive criticism well, but don't over-apologise: http://changingminds.org/techniques/assertiveness/recieving_criticism.htm

Interruption can be necessary. Learn when to interrupt, and how you can interrupt politely (it can be done, belive me!) http://changingminds.org/techniques/conversation/interrupting/interrupting.htm

Learn the do's and don't of negotiating here: http://changingminds.org/disciplines/negotiation/negotiation.htm

I'd love to receive feedback, as long as it's constructive and respectful. Feel free to add any more links related to negotiation, resolving conflict and assertiveness. Let me know what you think! :)