Someone could steal your identity very easily by:
- stealing your wallet
- filling out a change of address form for you and collecting your mail
- snatching your unshredded pre-approved credit slips from the trash
- ordering unauthorized credit reports on you by posing as a potential employer or landlord
- looking over your shoulder at phones and ATMs to gather PINs (sometimes with binoculars or listening devices)
- using phony telemarketing schemes to con you into giving your personal data
- phishing -- that is, attempting to get you to provide your personal or financial information by responding to Internet pop ups or spam
- getting personal information from you by emailing bogus job offers and requesting that you provide certain information
- illegally tapping a computer at a business to which you have provided information or by which you have been granted credit (this is often done by dishonest or disgruntled employees), or
- gathering sensitive information and using it as a way to extract revenge (this is usually done by a former friend, lover, roommate, or coworker -- and it's more common than most people realize).
To learn how to protect yourself against identity theft, see Nolo's article Preventing Identity Theft.