Disable messages upon loading a package

Just use suppressMessages() around your library() call:

edd@max:~$ R

R version 2.14.1 (2011-12-22)
Copyright (C) 2011 The R Foundation for Statistical Computing
ISBN 3-900051-07-0
Platform: x86_64-pc-linux-gnu (64-bit)
[...]

R> suppressMessages(library(ROCR))
R>                                               # silently loaded
R> search() 
 [1] ".GlobalEnv"         "package:ROCR"         # it's really there      
 [3] "package:gplots"     "package:KernSmooth"
 [5] "package:grid"       "package:caTools"   
 [7] "package:bitops"     "package:gdata"     
 [9] "package:gtools"     "package:stats"     
[11] "package:graphics"   "package:grDevices" 
[13] "package:utils"      "package:datasets"  
[15] "package:methods"    "Autoloads"         
[17] "package:base"      
R> 

library(ROCR, quietly = TRUE) might be a more elegant option.

Use suppressPackageStartupMessages, see the answer by MehradMahmoudian. For completeness, adding here examples of usage:

For one library, use suppressPackageStartupMessages(...), for example:

suppressPackageStartupMessages(library(ggplot2))

For multiple libraries, use suppressPackageStartupMessages({...}), for example:

suppressPackageStartupMessages({
    library(ggplot2)
    library(ggdendro)
})

SEE ALSO:
Suppress package startup messages

Dirk's answer suppresses all messages and is not specific to messages that is generated while loading packages.

The more accurate solution to the asked question is:

suppressPackageStartupMessages(library(THE_PACKAGE_NAME))

A bit more detailed explanation can be found here