Take good heed to yourselves. This is quoted from Deuteronomy 4:15, word for word. The Hebrew is, take heed exceedingly to your souls; but the meaning is either "as you value your lives" (Gesenius), or "with all your soul" (Keil).
The former appears preferable. A third interpretation, however, "guard your souls diligently," is suggested by a comparison of Deuteronomy 4:9, Deuteronomy 4:15.