Kudos to you for recognizing your signing skills and vocabulary could use some bumping up and for being willing to develop more skills.
My favorite book for learners is "Joy of Signing." The signs are ASL based and pretty standard across the country. You may also contact Galludet for resources. There are DVDs that target learning signs about different, specific subjects. They are available for loan for the cost of shipping, and used by interpreters and deaf persons alike to expand vocabulary.
The best way, of course, is to hang with Deaf people. Is there a Deaf club in your area? Do any junior colleges, churches, community centers offer classes (where they often waive a cost if they know you have a need and it's not just some "cool" phase)?
There is a guy on you tube who ranches and explains a lot of ranch and horse related stuff. He signs slower than what adults would normally sign to each other in conversation.