I usually abbreviate various forms of the word "synchronization" with the spelling "sync". It just looks right to me, perhaps because it seems to be the more common spelling---the iTunes command, the band 'N Sync, etc. I've also seen it spelled "synch", but that looks a little weird and less aesthetically pleasing to me.
The linguist in me, however, realizes that the root is χρόνος ("time"), and the Greek letter chi (χ) is always transliterated into the Roman alphabet as ch. So it seems that one should preserve the final "h" in the abbreviation. "Synch" is more correct...but I still don't like the way it looks.
I feel torn between aesthetics and linguistic purity.