Excellent article, and vital information. The less time you spend on toxic (and unprofitable) customers, the more time you can spend on good (and profitable customers). Recognising toxic customers is a vital skill.
Warning signs (see the article for more details):
- Disrespectful or Abrupt
- Asks for a Discount (With No Reason)
- Multiple Contacts, Often Through Multiple Channels
- Unrealistic Expectations
- Multiple Questions that Can Be Answered from Your Website
The article also describes an amusing encounter with a toxic customer, recognised early, and, unlike many such stories, it actually finishes well.
Update: excellent response from Joel Spolsky on HN.