High-performance cluster networks achieve very high throughput thanks to zero-copy techniques that require pinning of application buffers in physical memory. The Open-MX stack implements message passing over generic Ethernet hardware with similar needs. We present the design of an innovative pinning model in Open-MX based on the decoupling of memory pinning from the application. This idea eases the implementation of a reliable pinning cache in the kernel and enables full overlap of pinning with communication. Performance evaluation shows that both these optimizations bring interesting throughput improvements.