Previous research has demonstrated that the scope of speakers’ planning in language production varies in response to external forces such as time pressure. This susceptibility to external pressures indicates a flexibly incremental production system: speakers plan utterances piece by piece, but external pressures affect the size of the pieces speakers buffer. In the current study, we explore internal constraints on speech planning. Specifically, we examine whether individual differences in working memory predict the scope and efficiency of advance planning. In our task, speakers described picture arrays to partners in a matching game. The arrays sometimes required speakers to note a contrast between a sentence-initial object (e.g., a four-legged cat) and a sentence-final object (e.g., a three-legged cat). Based on prior screening, we selected participants who differed on verbal working memory span. Eye-movement measures revealed that high-span speakers were more likely to gaze at the contrasting pictures prior to articulation than were low-span speakers. As a result, high-span speakers were also more likely to reference the contrast early in speech. We conclude that working memory plays a substantial role in the flexibility of incremental speech planning.