I do it like this because the asdf central-registry variable (which I can't write here because of asterisk problems) is supposedly deprecated:

  (asdf:initialize-source-registry
    (:SOURCE-REGISTRY
     (:EXCLUDE "exclusion-string1" "exclusion-string2")
     (:TREE #P"/Users/me/Lisp/my-repos/")
     (:TREE #P"/Users/me/Lisp/quicklisp-repos/")
     (:TREE #P"/Users/me/main-project/src/")
     :IGNORE-INHERITED-CONFIGURATION))