For septatonic scales I use the following mnemonic: there are 14 notes in 2 octaves and they are distributed among 5 strings. 14=4*3+2 so there's one string with 2 notes and 4 strings with 3 notes. And it always goes like

OO-O

OO-O

O-O

O-OO

O-OO

The semitone steps seem to "gravitate" towards the 2-note string. So just loop this pattern around and you get all possible septatonic scale shapes (modulo the offset caused by shorter distance between G and B strings). Also the second note of the two-note string is always a minor scale root.