Support to political parties is perhaps the most difficult, and most criticized, form of democracy promotion. Despite this, there is relatively little research identifying how it might be made more effective. This policy paper draws on the body of practice accumulated by UK political parties, through programmes funded via the Westminster Foundation for Democracy, to help fill that gap. It examines what their distinctive approach to political party strengthening contributes to democracy promotion and identifies where these approaches work best. The evidence suggests that the sister-party model – a model centred on relationships between parties with similar ideological positions – has value, but that it would be more effective if it were deployed more strategically. When adopting this model, democracy promoters should be more selective about who they work with, where they work, and the kind of work they do.