Currently there isn't an easy way to add links to internal contents from the WYSIWYG editor (adding links is easy, but is oriented towards external links, without helping finding your own contents. You have to surf between differente windows or tabs searching the content urls from your own blog to insert a link to itself)
The idea: To date, in the wysiwyg editor, when inserting a link to a text, pop's up a layer where to add the links to the text. That layer could have a second tab where one can select from already created posts, pages and categories to link to.
It could save some headaches when building a hiearchy manually or linking to previous posts you don't exactly remember where they were or if you just are a little lazy like me. :)