In many Wireless Sensor Network (WSN) deployments, the communication medium is shared with other interference sources, such as WiFi, Bluetooth, and Microwave ovens. The performance of WSN Medium Access Control (MAC) protocols is known to be reduced under these conditions, including diminished packet delivery as collisions increase. The design of current MAC protocols, which cater for low energy and reliable packet delivery, perform poorly in interference conditions. This is due in part firstly, to the fragility of the sender/receiver rendezvous, and secondly, to the channel arbitration and collision recovery approaches being agnostic to the cause of packet loss. We analyse the operation of one particular MAC protocol, RI-MAC, and consider its behaviour under interference. Following this, three mechanisms are proposed to bolster interference resilience: Resilient Probes, Resilient Channel Arbitration and Resilient Data transmission. These mechanisms are designed around RI-MAC, however are broad enough to be applicable to most other WSN MAC protocols.