  We study numerically the formation of long-lived transient shear bands during
shear startup within two models of soft glasses (a simple fluidity model and an
adapted `soft glassy rheology' model). The degree and duration of banding
depends strongly on the applied shear rate, and on sample age before shearing.
In both models the ultimate steady flow state is homogeneous at all shear
rates, consistent with the underlying constitutive curve being monotonic.
However, particularly in the SGR case, the transient bands can be extremely
long lived. The banding instability is neither `purely viscous' nor `purely
elastic' in origin, but is closely associated with stress overshoot in startup
flow.
