backward, backwards

'Backward' is an adjective. 'Backward' and 'backwards' are adverbs. Both are acceptable. I tend to use 'backwards' exclusively when I need the adverb form. Consistency is important. More important is never using backwards as an adjective.

Scarlett volunteered for the dangerous mission. Hawk, being a chauvinist, shot her a backward glance. (adjective)

Walking backwards in the snow will confound anyone trying to follow you. (adverb)