Acrobat 8 Professional's Redaction feature makes it easy to highlight text or images and then permanently remove them from the PDF file. This feature was created for scrubbing sensitive information from legal documents, but also comes in handy for just "whiting out" information you do not want to include in a final PDF.
Here's a trick I ran into a few days ago: A client was trying to redact text from some emails that had been converted to PDF. There was insufficient space between lines, so the "Mark For Redaction" tool kept selecting text in the lines above and below the text that was supposed to be selected. A workaround for this problem, and a way to make more precise selections, is to hold down the Command key (Mac) or Ctrl key (Windows) while you use the Mark For Redaction tool. This will force the tool to select only text that is "touched" by the rectangular selection boundary that you draw.