I have about 1500 photos in my camera roll. Because of this, my iPhone has about 2GB worth of photos. I would like to have only the most recent 200 photos on my iPhone and the rest on my Mac. How can I do this?
I don't have an answer to your specific question--keeping the 200 (exactly) most recent photos on your phone, but I can give you a suggestion that may satisfy the spirit, if not the letter, of your request.
Connect the iPhone to your Mac and load the photos into iPhoto (or Aperture if you prefer). Choose the option to delete the photos after they've been loaded into iPhoto. This removes the photos from your camera roll.
In iTunes, set your iPhone to sync to selected Albums albums... and include events from the last three months. (Other time frames are available). You can also include albums, events or faces that you want to always keep on the phone. (for example, I have an album that contains a variety of photos of my niece and nephew from various events.) You can create Smart Albums that include photos meeting defined criteria, for example photos of my nephew taken within a certain date range AND containing the keyword "sailing". This will put a selection of recent (and special) photos into your iPhone Photo Library (and into the various albums within that Library). All of the photos are kept on your Mac (and should be part of your backup scheme) but the defined subset, plus the contents of your camera roll, will be on the iPhone.
As noted above, you can do the same thing using Aperture if that's your preferred photo management solution. iCloud can also help to automate the transfer of photos from iPhone to Mac, but you need to run iPhoto from time to time and I like to move my pictures into titled projects. (I'm an Aperture user.)
Again, it won't give you exactly 200 pictures, but it will give a subset selection of recent--or desired--photographs.