A large body of research has revealed that social support helps buffer the negative consequences of stressful life events. Yet research also suggests that social support exchanges involve complex interpersonal dynamics. Using in-depth interview data from 76 parents of paediatric cancer patients in the US, the findings demonstrate that parents experienced difficulty sustaining support after the diagnosis crisis, uncertainty about how to ask for the support they needed and challenges managing support efforts. Given these complexities in negotiating social support, this article examines the mechanics of effective social support exchanges. The findings show the importance of the nature of the pre-existing relationship between the support recipient and provider, as well as the interactions throughout the period of support. For example, parents found support efforts that involved frequent interactions and involvement in families’ daily lives (such as help with childcare) most effective when the support provider was a close network member. In contrast, support offered from members of extended networks was most effective when the effort required little marshalling from the parents, did not need to be asked for and did not intrude in the families’ private lives. These findings contribute to the medical sociology and social support literature by analysing the conditions under which effective support efforts are marshalled.