When participants switch between relevant stimulus dimensions in speeded classification tasks, task-switching cost is reduced by advance preparation. Previous studies in which speeded classification tasks were used have suggested that this effect results from attending to the relevant stimulus dimension. Because selective attention to the relevant stimulus dimension in same-different judgments is relatively poor (e.g., Santee & Egeth, 1980), it was predicted that advance task preparation for a shift in the relevant stimulus dimension would be compromised. This prediction was borne out in two experiments comparing dimension shifts (shape vs.fill) with task rule shifts (same? vs. different? and shifts in the mapping of right-left keys to yes and no responses (yes-no vs. no-yes). The results indicate that advance attentional selection of the relevant dimension is an optional preparatory strategy in task switching, employed only in conditions enabling flexible refocusing of attention.