First, use help.search() or the shorthand ??. This will search the help files of installed packages. I often find I have a package installed that does what I want; I just haven't used it before.
Next, use the findFn function in the sos package. This function searches the help pages of packages covered by the RSiteSearch archives (which includes all packages on CRAN). These are ordered based on a relevance score, so the top few packages on the list are probably the most useful.
To look even further afield, use RSiteSearch() which will send your search to R site search. As well as CRAN packages, this covers the R-help mailing list archives, help pages, vignettes and task views.
Still no luck? Try Rseek.org. It covers more sites.
Finally, if all else fails, ask here on StackOverflow or send your question to the R-help mailing list.
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