If the code compiles, I often just wrap it in if (0) { ... }. If it doesn't and you're commenting it out temporarily, using the POD style is your best bet; just keep in mind that it will feel a little awkward. If it's commented out longer-term, I would put # at the beginning of all the lines, as it's less likely to confuse people (including you!) looking at the code later. Hopefully your editor makes #-ing out a bunch of lines pretty painless; if not I might consider using POD format anyways.

That's just my personal style; I'm sure there are other ways of doing things with their own advantages and disadvantages.