I know a lot of educators like to number their documents by units, standards, and other numerical values, but hate it when files 10 and 20 come before 1 and 2.
The fix is to add leading zeros so that all your numbers are the same number of digits, e.g. 01 and 02. I also like using punctuation as separators, e.g. 01.02_lesson-name for the second doc in unit one. (This might be a leftover from when filenames with spaces would get corrupted, but I think it gives a clean and consistent look, making files easy to sort and find.)
What naming conventions do you use?