Are you copying an pasting text from a word processing program like Microsoft WORD? If so the enable this setting > Dashboard > Settings > Writing
__WordPress should correct invalidly nested XHTML automatically
See here please http://en.forums.wordpress.com/topic/the-paste-as-word-button-has-been-removed-from-the-visual-editor?replies=6
The paste from WORD button will not be restored. The paste as plain text button eliminates all code. http://en.support.wordpress.com/visual-editor/#pasting-text
Blog editors and word processors are not the same and do not work the same way. The web standard is one blank line only, so you cannot add more blank lines by pressing return repeatedly like you would do with a typewriter or in a word processing application. If you want to learn how to format and space your posts and pages see: http://onecoolsitebloggingtips.com/2011/09/13/wordpress-formatting-and-spacing/
When you create a new post, always make sure the Format tool is set to Paragraph before you start typing. It's in the Visual editor Row 2 first position "style". See the illustration here http://en.support.wordpress.com/visual-editor/#row-2 If you forget to do that then edit the post, highlight all in the Visual editor, select Paragraph from the Format tool, switch editor to Text then switch back to Visual and click Update.
Or use Windows Live Writer