What does Paul mean when he says Jesus "emptied himself"? In Philipians 2:7 Paul writes:


  but emptied himself, by taking the form of a servant,[a] being born in
  the likeness of men (ESV)


I always understood this to mean that he was humble enough to come into our world as a man to die on a cross. Something that I read recently though suggested that Jesus gave up some of his Godly characteristics to become a man (I'm not sure what charcteristics). This seems wrong to me as I always understood Jesus adds humanity to his Godliness, not removing Godliness to become human.

Is there a common protestant understanding of what "emptied himself" means in this verse?
The King James translates the same verse as this:

But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men:

Of course, there are likely to be several different meanings attributed to this, but the most common is the one that you posted - that He took on the humble nature of a servant.

Barnes' notes on the Bible says this:  (Emphasis mine where added)


  But made himself of no reputation - This translation by no means
  conveys the sense of the original According to this it would seem that
  he consented to be without distinction or honor among people; or that
  he was willing to be despised or disregarded. The Greek is ἑαυτον
  ἐκένωσεν heauton ekenōsen. The word κενόω kenoō means literally,
  to empty, "to make empty, to make vain or void." It is rendered: "made
  void" in Romans 4:14; "made of none effect," 1 Corinthians 1:17; "make
  void," 1 Corinthians 9:15; "should be vain," 2 Corinthians 9:3. The
  word does not occur elsewhere in the New Testament, except in the
  passage before us. The essential idea is that of bringing to
  emptiness, vanity, or nothingness; and, hence, it is applied to a case
  where one lays aside his rank and dignity, and becomes in respect to
  that as nothing; that is, he assumes a more humble rank and station.
  In regard to its meaning here, we may remark:
  
  (1) that it cannot mean that he literally divested himself of his
  divine nature and perfections, for that was impossible. He could not
  cease to be omnipotent, and omnipresent, and most holy, and true, and
  good.
  
  (2) it is conceivable that he might have laid aside, for a time, the
  symbols or the manifestation of his glory, or that the outward
  expressions of his majesty in heaven might have been withdrawn. It is
  conceivable for a divine being to intermit the exercise of his
  almighty power, since it cannot be supposed that God is always
  exerting his power to the utmost. And in like manner there might be
  for a time a laying aside or intermitting of these manifestations or
  symbols, which were expressive of the divine glory and perfections.
  Yet,
  
  (3) this supposes no change in the divine nature, or in the essential
  glory of the divine perfections. When the sun is obscured by a cloud,
  or in an eclipse, there is no real change of its glory, nor are his
  beams extinguished, nor is the sun himself in any measure changed. His
  luster is only for a time obscured. So it might have been in regard to
  the manifestation of the glory of the Son of God. Of course there is
  much in regard to this which is obscure, but the language of the
  apostle undoubtedly implies more than that he took an humble place, or
  that he demeaned himself in an humble manner. In regard to the actual
  change respecting his manifestations in heaven, or the withdrawing of
  the symbols of his glory there, the Scriptures are nearly silent, and
  conjecture is useless - perhaps improper. The language before us
  fairly implies that he laid aside that which was expressive of his
  being divine - that glory which is involved in the phrase "being in
  the form of God" - and took upon himself another form and
  manifestation in the condition of a servant.


In short, per the Barnes' notes, He did not lay aside any of His power or Godly characteristics, He merely did not exercise them.

Gills' commentary says this:


  ...He lost nothing of what he had; the glory of his divine nature was
  covered, and out of sight;...


Vincent's Word Studies also agrees:


  The general sense is that He divested Himself of that peculiar mode of
  existence which was proper and peculiar to Him as one with God. He
  laid aside the form of God. In so doing, He did not divest Himself of
  His divine nature. The change was a change of state: the form of a
  servant for the form of God. His personality continued the same. His
  self-emptying was not self-extinction, nor was the divine Being
  changed into a mere man. In His humanity He retained the consciousness
  of deity, and in His incarnate state carried out the mind which
  animated Him before His incarnation. He was not unable to assert
  equality with God. He was able not to assert it.


as do Wesley's notes


  Though he remained full, John 1:14, yet he appeared as if he had been
  empty; for he veiled his fulness from the sight of men and angels.


Most of the commentaries that I can find agree with your original understanding of the phrase.  He retained all His power and glory, he merely chose not to exercise it.

These, and a whole slew of other commentaries are available at http://bible.cc/philippians/2-7.htm