DO SERVE AS A REVIEWER for journals, particularly top journals. Treat this job seriously. You will see much junk being submitted and appreciate why some journals reject 80% or more of their submissions. You will develop an aesthetic for what is good and what is not. You will correspond with some powerful people. When you do get a good paper to review, you will receive much earlier knowledge of an important new development. The information gained is worth more than the time you take reviewing.