Plate your cells in a volume of 50ul (I am assuming you are performing the assay in a 96 well format). Then you can add the desired drugs either the same day or the next day in 50ul (I prefer the next day, to allow cells to adjust to the new plate environment and reduce stress influences). Remember to perform this assay in a round bottom plate, rather than a flat bottom plate, as one would with adherent cells. After the desired incubation time, you can then add 100ul of the solvent, SDS indeed works very well, but you can also use MTT solvent. Make sure to resuspend the cells in the solvent and incubate on a shaker, to disrupt all crystals formed by the incorporation of the MTT reagent. Then you are ready to read your plate.
Remember, the reason why you can use an MTT assay for non-adherent cells, is because you do NOT need to change media before adding the reagent, or before reading the plate. This is why this is an easy method for non-adherent cells.