To preserve history and to make it easier to undo changes, I usually
merge individual changes.  This may sound difficult, but in fact it's not.
The most important thing is to get a feeling for it and find a good
balance between merging time and testing time.

Here's how I usually do it:

* pick a day where you are not too busy doing other stuff

* Look at the current merging status:

  $ cd mcs/gmcs
  $ svn propget gmcs .

  For the rest of this document, let's assume everything till r49341 is
  merged.

* Do a

  $ svn log -r 49341:HEAD .. /mcs/ | a2ps

  to print the SVN history.

  Read over it, completely to till the end.  Look out especially for
  reverted changes.  If you encounter a reverted change, take a note
  at the revision which introduced the change - so you know later on
  that you don't have to merge that revision.

* Make sure you have a working tree `make bootstrap-world' works; don't
  touch that tree during the merging.  I usually do the merging in a
  separate tree, so I can continue working while the tests are running.

* After that, I start merging like this:

  $ cd /work/rohan/mono/mcs
  $ cp -a gmcs gmcs.0		# make a copy of the working version
  $ cd gmcs
  $ svn merge -r 49341:49488 ../mcs/
  $ make			# make sure it compiles; this can save
                                  you a lot of time later !
  $ cp -a . ../gmcs.49489       # make a copy
  $ svn merge -r 49488:49497

  ....

  You usually get conflicts in the ChangeLog; make sure to resolve them
  before going on.

* If there are several consecutive small changes from the same person,
  I usually merge them at once.  The same applies if someone made trivial
  changes like fixing a typo; I include them in the previous revision.

* If the next revision contains a larger or structural change (patches
  from Marek and Hari are a good example), I stop the merging and run
  the tests:

  $ make -C /work/rohan/mono bootstrap-world > logfile 2>&1 &
  $ tail -f logfile

  Then I go work on something else.  It usually takes about 20-30 minutes
  for the tests to run.

* If the tests succeeded, I commit all the previous revisions:

  $ cd ../gmcs.49488
  $ mv ChangeLog ChangeLog.new ; svn up ; mv ChangeLog.new ChangeLog
  $ svn ci -m "**** Merged r49487-49488 from MCS ****"
  $ cd ../gmcs.49497

  The `mv ChangeLog' is a small trick: SVN would otherwise mark the file
  as conflict.  The `svn up' is neccessary as it'd otherwise report a
  "working directory out of date" error.

* After that, I take a note on my paper that I merged everything till
  this revision and clean up:

  $ cp /work/gondor/mono/mcs/gmcs
  $ rm -rf ../gmcs.*
  $ cp -a . ../gmcs.0

* When done, update the merging history:

  svn propedit gmcs .

That's it, basically.
Martin