How can I contribute to base R in small ways?
Send (or CC) to r-devel. Traffic is quite high on r-help, and things can be overlooked there.
File a bug under the wishlist category detailing the improvement you would like to see.
Having filed the bug, try to provide a patch against the R code and or documentation as appropriate. I've done this before where there was a problem or infelicity in R, supplied a patch and a fix to the help files/manual and had the changes accepted (after suitable modification) by R Core.
If it is an addition to the R code base, you are going to have to show that there is a real pressing need for the addition. Basically you are asking R Core to maintain your code in perpetuity, and they are unlikely to do that unless you can demonstrate a need.
If it is an addition, look for a popular R package that does similar/related things and suggest to the package maintainer that they include your function. That way you don't need to start a whole package for something simple but contribute your code. There are several, popular, *misc packages on CRAN for example.
If you want to contribute fixes to the R documentation and/or manuals, provide patches to the sources. You can find the sources at svn.r-project.org/R
Hopefully that gives you some ideas. Patches and code always help!
How about patches to existing packages?
How about open bug reports on packages? R-Forge projects don't seem to use the issue trackers much, but some folks on the RPostgreSQL team I'm on enabled it (where it is hosted on Google Code), and it has been helpful -- see here. And we had a really useful inflow of fresh blood with a rocking new developer from Japan, probably in part because of the visibility of the project there.
In essence, try to find a project / group / team to become acquainted with and join. In that sense, this is just like any other Open Source project. The r-devel list (gmane view) is a good place for R development in general.
The R Core team, on the other hand, is a little more closed and per invitation only and unlikely to change. So be it, for better or worse. It has worked so far, and hence I am not among those who bemoan this loudly.