When I googled "saving mustard seed," I got a whole mess of sites talking about faith.
This is not about that. At all.
Although, I remember talking about mustard seeds, and how they are so so so tiny and they grow into such a huge plant/shrub/tree. Represents faith and all that. Mustard plants do get pretty big. My mustard greens have gone to seed, and are about as tall as I am. I have yet to see the mustard tree, but I guess a certain someone wasn't planning on making his book make sense decades, let alone centuries, after it was written. The whole watchmaker making a watch, then not giving anyone decent instructions on how to wind it and all that.
However, I found a good site or two on saving seeds. scroll down for mustard. There's also this one, and here's a neat one that lists how long the seeds you save are viable. This site has a bit of historical info.
Man, time flies. Got 15 minutes to get ready for work!
What's cross polinated with what.