Synching selected music to iPhone

I'm having a problem synching selected music to my iPhone from my iMac. I'm currently using the latest version of iTunes. I have a large music library on iTunes. I used to be able to check box the tracks that I wanted to export to the iPhone. I recently started to do an iTunes synch, and I found that all of my tracks were being exported. Much more than I wanted to export.

I haven't be able to find a way to specify which tracks to export.

Thanks!