Watch the Standard Walking Bass online guitar lesson by Mimi Fox from Flying Solo
Chord substitution will spice up your bass lines by giving a sense of shifting harmony. For example, if the song calls for a minor chord, use the relative major instead and choose a bass line from the major chord instead of the original minor chord. A slight shift like this will add interest to what your playing (for you, and the listener!) and also lead you into different areas of the neck where you can explore new combinations. Be creative!