This may be too late, but what about running a piece of wood through the bottom of the cage (I'm guessing here that it may be one of those that is made of a section of fencing?) and securing that to the cage with some wire? A spare 2 x 4, some larger branches you've been meaning to prune anyways, something of that sort. That would effectively broaden the base of the cage (as well as add weight down low, thereby lowering the center of gravity for the whole thing).

I went the route of trying to guywire my cages last year. That worked for awhile, but I should have anchored the ends better (spare bricks would do nicely, I think).

Hope you find something that works. Let us know if you do, as none of the rest of us want our tomatoes knocked down in a storm either!

P