Some guys swear by doing just a quick 30 secs or so worth of stretching every time they go to take a leak - providing that you have a little privacy of course. I've never done this and have no idea whether it holds any merit or not, but if you're time-crunched it couldn't hurt to try it out for a while. I think they call it 'piss pulls' or something like that, maybe try a thread search.
There are apparently also all manner of stretching devices and such that can be worn during the day, but again, I've never tried or even looked at any of that stuff too seriously.