Files to put on SSD:
Files that are frequently read but not written (OS files, Programs that are used often)
Files you want to load quickly (OS files, Programs that are used often)
Files to NOT put on a SSD:
Big files that are frequently written (If you frequently download HD videos, put the download directory on a HDD)
Files that don't need to load quickly (for example videos, music, photos)
Very big programs that you don't have the space for on the SSD (games)
If you have a lot of RAM you could also put the Temp directory on a RAMDisk, force your browser to keep the cache in RAM, extract archives directly to the destination (instead of to Temp, then move to destination) etc. to minimize write access on the SSD. But that's just for hardcore users, normal users shouldn't really have a problem with an SSD dying too quickly nowadays.