my_folder
|-- my_sub_folder
|   |-- my_file_1.txt
|
|-- my_file_2.txt
|-- my_sub_folder_2
|   |-- my_file_2.txt

# cd my_folder
# find . -type d | xargs chmod 1777

# ls -ld my_folder
drwxrwxrwt 4 root root 4096 2014-12-01 18:52 my_folder

$ cd my_folder
$ echo foo > user_file     # ok
$ rm user_file             # ok
$ rm  -f my_sub_folder/my_file_1.txt
rm: cannot remove `my_sub_folder/my_file_1.txt': Operation not permitted