Here's a trick that really works with most car remotes. Now, you're not going to believe it at first, but try it and you'll see.

If you touch the remote to your chin when you activate it, it will increase the range. It sorta uses your skull as a bigger antenna.

I haven't tested it with the X5 style remotes, but I have tested it with Ford Focus, Ford Taurus, and Z4 keys/remotes, and using the lock/unlock function it clearly increases the range.