How do I skip certain directories in "du" output?

My initial response was that you'd want to just pour the output through a grep -v, or, if you want to get fancy and use a regular expression, grep -vE. Like this:
  du * | grep -vE '(/usr/|/bin/|/mnt/)'
which would give you an exhaustive output of disk usage by directory, but would skip any directories that matched the pattern /usr/ or /bin/ or /mnt/

It answered the question, but the intent of the person, however, was to also speed up the disk usage calculation itself; what he really wanted was to mask the directories within the du command. As he asked: "Would that skip retrieving the disk usage for any of those directories, or simply skip displaying the disk usage? The main problem with retrieving the information on those directories is that it can take a lot time."

It doesn't skip getting the information, you're right. If you want to do that, then you need to explicitly list the directories you DO want to know about. One way to do that, though, is to drop a list of all directories in the current location into a file, then edit it to be what you want. Something like this:

find . -mindepth 1 -maxdepth 1 -type d -print > .localdirs

$ du -s $(cat .localdirs)

$ du -s `cat .localdirs`