[go] Organizing a multiple-file Go project

jdi has the right information concerning the use of GOPATH. I would add that if you intend to have a binary as well you might want to add one additional level to the directories.

~/projects/src/
    myproj/
        mypack/
            lib.go
            lib_test.go
            ...
        myapp/
            main.go

running go build myproj/mypack will build the mypack package along with it's dependencies running go build myproj/myapp will build the myapp binary along with it's dependencies which probably includes the mypack library.