Two questions, two answers;
Cantus firmus: To my knowlegde cantus firmus is essentially a relatively simple melody, that can be placed in whichever voice, including the bass. This has meaning for lots of modern songs where the core is a fixed bass pattern. But since you were asking about harmonizing songs, this does not apply here.
Counterpoint for the contemporary bass player (in a nutshell):
1. Deekay911 is on the dot with contrary motion. Generally, it is less nice to move all voices in the same direction. The role of the bass often is to go up when the melody goes down and the other way around.
2. Like I said, avoid forbidden parallels
3. Avoid second inversion (5th as bass note) on strong beats (unless the entire audience wears boots).
4. If the chords are progressing really nicely, try to add a sense of direction by moving towards a real milestone of the progression taking whole a/o half steps (like in a scale), preferably landing on the root at that milestone
That should do it!