1. list the notes from all chords in the progression for the song
2. look for a scale that includes all those notes
3. If you can't find one, switch scales while playing solo over the part of the song with the odd chord to reflect the notes within the off chord
4. to emphasize notes for the currently playing chord by starting, finishing and taking downstrokes (the 1 and 3 in 4/4 time) on notes within that chord
5. pay attention to which chord is currently playing and try to add your improv so that it doesn't interupt the rhythm of a song. Runs and licks should start and stop within the flow of the song.