Although this probably sounds a little funny to us today, this saying was most likely pretty common in Jesus' day. There are several places in the scripture where Jesus used common terminology to apply truth to a given situation. In this case, he was sending out the 70 followers to teach and evangelize.
The image that Jesus is producing here is that we need to be deliberate and calculating with our moves. We don't just want to rush in and do things just because we can or because everyone else is doing them. We want to think before we act.
The other image is "innocent as doves". This one is a little easier to understand. Innocence is often characterized as trusting, loving, submissive, and oblivious to the evils of this world.
Jesus is projecting a balance here. We need to be aware of our surroundings and move cautiously and calculating. But we also need to trust God and be submissive to his will while all along keeping from being entangled in the affairs of this world.
Martin Luther King, Jr said, "“We must combine the toughness of the serpent with the softness of the dove, a tough mind and a tender heart.”
Prayer Thought: Dear Lord, God Almighty, I praise you for the opportunity to serve you to this fellowship of believers and the community here. I ask that you will help me to be as shrewd as snakes and as innocent as doves. Give me the strength, courage, and the words to say as I reflect your glory. I pray this prayer in the name of Jesus, your Son and my Savior, Amen.