You need error detection and recovery.

As described earlier, a CRC gives you error detection, and you can easily do error recovery by retransmission:
http://www.8052.com/forum/read.phtml?id=92118

As Erik said, you need to set your block size so that retransmission is not too much of an overhead.
You could even do this dynamically - reduce the block size when the retransmission rate is high, increase it when it's low.

To avoid complete loss of an entire packet, you need some sort of sequence numbering - it could be just a single bit, if you only ever allow 1 outstanding message (window size = 1).

I haven't looked into Michael's protocol - he's given the code, so check it out.