Another possible method-which I started using because I have a 'saga' whose multiple series are interleaved-is to assign the series number in the order in which I think they should be read, with the series name being the saga, then append the individual series to the title. Not good at screen captures so I'll give an example:
Forsyte Chronicles Forsyte Chronicles (1)
Forsyte Saga: Book 1 Forsyte Chronicles (2)
Modern Comedy: Book 1 Forsyte Chronicles (3)
Forsyte Saga: Book 2 Forsyte Chronicles (4)
etc. When sorting by Title it groups the series. When sorting by series it groups the saga in the proper order.
Assuming the saga does run in order then DopedAngel's suggestion of integers works well-but when the series are interwoven it just doesn't seem to work for me although I've tried using decimals for the series, like .1 = Forsyte Saga, but then I always seem to find some that I need to put 2 in the same series between ones in other series. Maybe 2.11 & 2.12 would put 2 Forsyte Saga books between a Forsyte Chronicle (2.0) book and a Modern Comedy (2.2) book. I think that would work but it just got too cumbersome for one that I ended up with about 4 different 'series' in the same universe.