From Lynch, Guide to Grammar and Style T
They're interchangeable. Toward is a little more common in America, and towards a little more common in Britain; but both forms are perfectly acceptable in either place. [Entry added 24 April 2006.]
From toward vs. towards - Wordsmith.org
James J. Kilpatrick, in his book Fine Print, says there is no discernible difference between the two: the British prefer towards; American usage calls for toward. Fowler, however, has a more complicated answer. He notes differences in pronunciation and usage (adjective/preposition). Says use as adjective is obsolete and, as preposition, the -s form is the prevailing one, and the other tends to become literary on the one hand and provincial on the other.
From Online Etymology Dictionary
Old English toweard "in the direction of," prepositional use of toweard (adj.) "coming, approaching," from to (see to) + -weard, from P.Gmc. *-warth, from PIE *wert "turn" (see -ward). Towards with adverbial genitive ending, was in Old English as toweards.
From American Heritage Dictionary
USAGE NOTE: Some critics have tried to discern a semantic distinction between toward and towards, but the difference is entirely dialectal. Toward is more common in American English; towards is the predominant form in British English.
From towards; toward;
I don't think I even notice whether I or another speaker is using the -s form or not. I'm sure I have slight a preference for the -s form as an adverb (in my mind, reserving the S-less form for adjectives ["forward movement"]), but I'm also sure that in my actual speech, the adverb comes out more or less at random either way.