how should I handle salary requirements in an interview!? why do I need to tell them how much I was making before. I am not a good negotiator when it comes down to salary

I interviewed three times at company that I wanted to work for....during two of the interviews, they asked me my salary requirement and I told them. They agreed, acted like it was OK and never mentioned the company's maximum salary allotted for the position. The salary I gave was $10K less than my previous permanent position. But when I received the offer letter, it was $5K less than what was agreed at the interview. So I asked about it (maybe it was an error or did the salary change)....I was told that was the budget for the position. I tried to negotiate and mentioned that nothing was said to me at the interviews that my given salary was out of the range. I tried to negotiate $2K - $3K less instead....but after my suggestion, I received an email that the offer was being rescinded. And an apology for the confusion but they decided to go in another direction. Later I sent an email apologizing if I was too forward with my negotiating skills and explained that I was still interested in the position. I have never negotiated for any of my previous positions....but I have been told that you never accept the first offer - you negotiate. And especially since the salary being offered was $5K less than agreed at interviews....I did not want to just accept what was placed on the table. I guess negotiating is not a good idea in this job climate.....but I didn't want to take a $15K paycut. Wrong move....

What do you do if you have two companies are about to offer you a position at the same time? Do you play one off of the other based off of benefits and salary?

The salary range listed on the job description is ($13.25 - $15.00) for an inbound customer service position. If the offer is $13.25 how can I negotiate for $15?