What temperature should mammalian B-Cells be stored at outside of the incubator?
For the settling there's little you can do except agitation and using a taller thinner tube and larger suspending volume. 

For the clumping however if the analysis permits it, adding serum to a [final] of 5% will help slow the clumping (in the short term). You can also filter out initial clumps by pushing the cells through a nylon mesh filter (they make these especially for flow-commonly used with fixed cells). I've analyzed cells on ice in dark probably as long as 6 hours after harvesting with little change in viability, however this is of course bad practice.