Watch the I iv/I online guitar lesson by Brad Carlton from Guitar Lab: Slash Chord Science
This lesson is based on the following chord progression: I iv/I in the street key of C. Theoretically this vamp functions in two different keys. The C chord could function as a I in the key of C or a IV in the key of G. The Fm chord could function as a ii in the key of Bb or as a i from the F melodic minor scale.
Learn the inversions and arpeggios for the two triads used in this progression and practice playing them over the jam tracks. Then add melodic moves by using the remaining scale tones of each applicable mode. I will demonstrate this concept in the video both in the rhythm guitar and lead guitar sections.
Your goal should be to have complete mastery of the entire fingerboard thus allowing you to move this arpeggio/scale map up and down the neck to accommodate any key. You also want to develop your ears so you can hear these slash chord harmonies as well as understand all the theoretical ramifications.