We use Subversion (SVN) to manage our code, and typically branch the code from 'dev' to 'prod' at launch time, with each project's tech lead determining the best way to manage the two repositories in parallel for subsequent development. SVN, like any decent versioning system, has branching and merging functionality, but until recently I had little success (and some disaster) trying to use it. So I used the cruder but tried-and-true method of locally rsync'ing dev to prod (with -rC for recursive and ignoring SVN metadata) and committing the changes to each separately. That mostly works fine, except it can be a very tedious process (using FileMerge to check every file and figure out which branch's version is correct); it makes SVN's logs very difficult to understand (with no clear history to a file and duplicate commits of everything); it's easy to make a mistake and mess up a branch; and it seemed unnecessary given SVN's built-in functionality. In some cases, the dev and prod branches got so out of whack that the developers had given up on merging them, and all development was done on local copies of the production branch, an inefficient (and dangerous) method that made branches pointless.

So on my latest post-launch project, I decided to figure out SVN merging. The first deployment process took 2+ hours with a splitting headache at the end; then 1 more lucid hour; now it's 5 minutes, so I do this even for extremely urgent fixes. svn help merge describes multiple ways to use the command; this is how I've gotten it to work.