Robert_Charlton - 7:39 am on Jul 25, 2013 (gmt 0)
On pages that might rank on a range of terms, I sometimes use a "150/250" arrangement, as I call it... 250 characters overall, with roughly the first 150 characters including the most-likely-to-rank terms. These 150 are incorporated into a self-contained thought to display as a sentence; and I play with the remaining 100 characters to add secondary targets.
Google seems to be good at spotting break points, particularly when they're indicated with punctuation. Semicolons can be useful to break descriptions; they're more flexible than periods or commas.
For the information of newbies who might be reading this... while the meta description doesn't affect rankings, the highlighted search terms in the results can attract the eye and increase click throughs, particularly when combined with an effective call to action or value proposition.