I absolutely agree with nmno - list your best credits first. A hiring PM is usually so swamped with resumes, that they might only read the first few lines. If you don't catch their eye, they move on to the next one. So be impressive right at the top of the page!
But as for your overall ogranization, this really depends on what job you're applying for. If you are trying to get quite specific work (concerts, opera, etc.), I would list your credits by category (plays, musicals and opera, events & concerts, etc) to show your diversity. You generally can't go wrong listing shows chronologically starting with the most recent - or even future - engagements first. If you don't have a lot of experience or have mostly small or school shows under your belt, you could do your best to list biggest/most impressive shows first, regardless of chronology or your position. It's my opinion that listing shows by your position (PSM shows, ASM shows, PA shows, Other tech, etc) is the weakest way to show your experience. But do always list the position you held when you list the show - this is essential information.
You should definitely check out the resume browser on this site to see how other SMs of all areas and levels of expertise arrange their credits for the most success.
