Plate pinches are good for grip work. I use them as well For further info on this kind of thing go to
http://www.dieselcrew.com and you will find all the grip info you could want.
In this case it sounds like your hands are actually sore already from your workout so I would advize not starting with plate pinches unless you are assured of plenty of time to recover (such as the weekend) because you are lible to just get you hands so sore you can't hold onto anything. And start light as you can and don't necessarily go for 30 seconds. Like Dave said you can overwork you hands but not only that but you can make matters worse by being too aggressive when you have bad hands. A simple hold may be better for you to start with...such as holding loaded dumbells for time...
Read up on grip work more.
The hands can be sort of a mirror of overtraining as well.