Making good use of HD + SSD combo on a new iMac
My first advice: don't fret too much over SSD lifespans or performance degradation. While it has been an issue (and still is), for almost everyone it's not likely to be an issue over the lifespan of your computer. Far more likely that you'll want to replace your iMac before the flash runs out of writes. Additionally, the Samsung and Toshiba drives Apple currently uses are are more in the reliability end of the spectrum, rather than sacrificing lifespan for huge speeds.

Secondly, to really get the most benefits out of the SSD, you'll want to put your OS and apps on the SSD, and big files like movies, music and photo libraries on the HD (if you keep the metadata files like iTunes Library.itl on the SSD, you get speed improvements opening big libraries, without having to store the big files there). The most noticeable improvements from SSDs aren't the huge sequential r/w speeds, but the smaller random access speeds, which you really notice for OS and app start times, and any memory paging to disk.