1. What size screen would you like, 5", 6", 7" or 8" or 9.7"? I will only use this as an example but if you live in an area that sells Sony Readers check out the size differences between the Pocket (5" screen), the Touch (6" screen) and the Daily (7" screen).
2. What kind of options would you like to have. Some of the options are touch screens, built-in dictionaries, note taking, collection/folder support, 3g/wifi book downloading, E-Ink vs backlit LCD.
3. What types of books will you be reading most. If you read a lot of technical books with pictures/tables/graphs then you may want to limit yourself to the 7", 8" and 9.7" screens. If you read mostly fiction/non-fiction then a 5", 6" or 7" screen may be more suitable as they are smaller, lighter and easier to carry around when travelling.
4. If you read fiction/non-fiction then take a look at the various ebooks stores and compare prices of the last 10 books you read/purchased. For some ebooks the prices will vary by store and could be a deciding factor for some. Using sites like Inkmesh
will provide prices amongst the various ebook retailers. Also look into whether your public library offers ebook borrowing as this may help eliminate the a few ebook readers that do not support library books.
Once you have specific criteria in hand we will help narrow down the choices for you. Hopefully you can find the ebook reader that fits your needs.