Assume that you have a directory under subversion control, that contains some files and tons of subdirectories, like that:
file1.txt file2.txt file3.txt dir1/ di
With svn 1.6, --set-depth is the answer.
--set-depth
With svn < 1.6, you can get sparse checkouts by abusing svn switch: Create an empty directory somewhere in your repository, and switch the subdirectories you don't need right now to point to that instead of their "real" URL.
svn switch
switch