This is a simple little trick, yet very effective.
[Pointer finger -1 Middle finger- 2 Ring finger-3 Pinky Figer -4]
With your chording hand place finger 1 on desired fret, atop finger 1, place finger 2 and mash down. Two frets UP the neck mash finger 4 across D,G, and B strings.
Ba-ding-a-chow-eeee!!! You should notice a nice, clean, 5 string barre chord on your fret of choice.
Do not get too comfortable with this technique though. We need to free up that pinky finger for fills and extra, fancy little tricks in the future. Once in a while, try the 5 string barre chords with only your pointer finger mashing down the frets.
One additional little trick I picked up along he way is to gently touch the low E(thickest) string with my pointer finger as I do a 5 string barre chord( A shape). By touching, not pressing the low E it mutes the string and prevents an unwanted note. Let the low E ring out if you're playing a 5 string barre chord E.
D (x 57775)
D# (x 68886)
F (x 8 10 10 10 8)
F# (x 9 11 11 11 9)
G( x 10 12 12 12 10)
G# (x 11 13 13 13 11)