Best practice to merge from branch to branch

Best practice to merge from branch to branch

We have a fairly typical SVN repository setup, with ^/trunk holding the
current stable version of our software, and development/bugfixes going on
in feature branches located under ^/branches/<feature>. Branches are kept
in sync with trunk, and once a branch is feature-complete, it has to pass
a series of tests before it is reintegrated into trunk.
Sometimes, however, I'm finished with a feature A and would like to work
on another feature B, in its own branch, before branch A can be
reintegrated into trunk. What is the best practice from getting features
from one branch in another, without "breaking" the history more than
necessary?
Just to clarify what I mean with "break": My aim is that when A is finally
reintegrated into trunk, and I do a merge from trunk to B, it should not
produce any conflicts and the "blame" of the work should still be
contributed correctly when I finally reintegrate B into trunk as well.
P.S.: This should work with svn <= 1.7, since we cannot switch to 1.8 yet.