This always seems to get me and I have to figure it out every time. Sometimes I have files with blank lines and I want to get rid of them. Here is how you do it in vim (one of the many ways, actually). First you want to get rid of whitespaces:
Note: I used a blockquote here because WordPress doesn't like two forward slashes in the sourcecode tag.
Then, you have to do this:
:%!grep -v "^$"
Of course, this only works in Linux, BSD, or any of the other Unices.