create .Rprofile in your home directory

.Rprofile

# No need to specify repos to install a CRAN package
cat("Setting R repository:http://cran.case.edu\n")
repo = getOption("repos") 
# set up the server from which you will download the package.
repo["CRAN"] = "http://cran.case.edu" 
options(repos = repo)
rm(repo)

# load BiocInstaller/biocLite when R starts
# you can now directly install bioconductor packages using biocLite('package-name')
# no need to load biocLite explicitly
cat("Loading BiocInstaller:\n")
library(BiocInstaller)