All that is required in order to view and download 1940 census images is a free Ancestry.com account, so no need to utilize that 14-day free trial just yet (if you haven't already).
Most states in the 1940 US Federal Census are also searchable and available for download free at FamilySearch. All of the names have been indexed, but a handful of states, including Texas (where all of my relatives were), won't be searchable for a few more weeks.
I managed to find about half of my 1940 ancestors by browsing the enumeration districts where I thought they would be living, but many were hard to track down. I now have a complete collection of all of my husband's and my own direct lines from the 1940 census schedule. Yay!
Like with previous indexed census images, the 1940 index does have errors due to illegible handwriting, fallible informants, and surely a few typos here and there. Also, spelling variations in first and last names may be an issue, though Ancestry.com does try to account for this situation.
If you are having no luck finding a particular ancestor, be sure to try searching instead for another relative who would have been living in the household. You can also just search for the surname within a specific area in order to uncover other relatives in the same area with a single search.
Also be sure to browse up and down census pages for relatives living nearby. I wasn't looking for my great-great grandmother yet, but stumbled across her living a few doors down from my great-grandmother's family.