Negative keywords are very important for pay per click advertising as they prevent your ads to appear on irrelevant keywords. Appearing your ads on irrelevant queries will increase impression and reduce click through rate hence reduce quality score and will increase cost per click just because not proper use of negative keywords. Even if you target on very generic terms and have large volume of local/global monthly search traffic the volume of irrelevant queries is much higher than your targeted volume which continuously affects your quality score and increasing your cost per click. Not using negative keywords is like as you are paying to not buy your products or services. So please remember negative (keywords) is positive in pay per click marketing.
How to research negative keywords
1) Same like when you research for campaign keywords when you think like a customer to find what queries a user can enter to buy your services or products same way think like a customer on what queries your ads should not appear
2) Use keyword tool to find relevant keywords for your primary negative keywords.
3) Checking search term report regularly to find irrelevant queries and add them as negative keywords.
Search negative keywords for campaign and ad group level because some keywords are relevant at campaign level and irrelevant at ad group level so choose your negative keywords carefully as adding your relevant keyword as negative can also loss of business.
So please give importance to negative as positive has no importance without negative 🙂