Hi Lex
I Haven't special technique to make exposures of 20' or more, my mount is a standard commercial Eq6, without modifications.
It's important to pay attention to details as mount stationament, set-up balance, strong focuser (avoiding banding)
Sbig ST2000XM is a dual-chip camera, often I guide with internal chip (absolutely when I use VMC200L F1950mm as scope), but not in this case.
With H-alpha and other narrowband filters, sometime, it's difficult to find a guide star and so, with this refractor of F800mm only, I also use an external guide system, other small refractor (62/520) not shiftable and Atik 320E as guide ccd.
As software for capture and guide Maxim DL5
I'm sorry for my bad english.
Roberto