Learn to automate. If you have a language you enjoy, learn the popular test automation tools in that language. Someone who can automate testing is far more valuable than someone who can only do manual testing. Along with automation goes all the usual best practice of writing software.

Read about testing - lots of inspiration and ideas to be had. Anything by James Whittaker. O'Reilly's Beautiful Testing.

(Please fellow HNers add your favorite books in response - would love to know what others recommend).