Jesus' command to the Sardisians to remember is similar to his command to the Twelve concerning the Lord's Supper. In that case the subject of recall was Christ himself, in this case it is what the Sardisians had embraced (received) and understood (heard) as believers. This is not the first time a NT writer used recalling that which had been experienced by believers as a means of correction. It seems that Christians forget what they've seen and heard, and which elicited and strengthened their faith in Christ at their own peril.
Instead, the Sardisians were told to guard (keep, watch over) what they had embraced and heard. Finishing one's work certainly progresses toward that end, but it also means revisiting, checking up on, remembering the things by which we stood and continue to stand in faith. The celebration of Communion can help with this, so long as it isn't seen as making the bread and wine instill some spiritual benefit of themselves, rather than being a means of remembering the Lord and what he's done to save us.
The Christians of Sardis needed to change their minds about and attitudes toward (repent) what they had already heard and known. It is my sense that human nature has a ready appetite for the fresh, that we are more intrigued by the new rather than what we have already been exposed to. We can be dismissive and even disdainful toward what was once fresh but is no longer on the cutting edge. Disregard for the foundational and worthy of continued attention can not only lead to works going astray but also faith going dry.
"Wake up!" Jesus commanded the Sardisians. Really, watchfulness is the practical outcome of wakefulness which seems to be the point Christ was making--pay attention to your ongoing faith walk with Jesus. The mention of his surprise appearance catching them unaware is reminiscent of the Parable of the Ten Virgins. Whereas there, sleeping per se was not cast in an irredeemable light (as it is here), the force there and here is the faithless lack of attention and concern some have toward what they have and are expecting in Christ.