Create a branch:

svn copy http://myserver.com/repos/myproj/trunk \
         http://myserver.com/repos/myproj/branches/myproj-branch \
         -m "Creating a private branch of /myproj/trunk."

Checkout a branch:
svn co http://myserver.com/repos/myproj/branches/myproj-branch

Merge my local copy of a branch with trunk

svn status
#make sure your local copy is in sync with the branch first.

#preview what will be merged:
svn merge http://myserver.com/repos/myproj/trunk --dry-run

#do the merge:
svn merge http://myserver.com/repos/myproj/trunk

svn commit -m "Merged latest trunk changes to myproj-branch."

# can also merge per revision:
svn mergeinfo --show-revs eligible http://myserver.com/repos/myproj/trunk
svn merge -r xxx:xxx http://myserver.com/repos/myproj/trunk
# or per 1 revision instead of range
svn merge -c xxx http://myserver.com/repos/myproj/trunk

Merge my branch with the trunk

cd ~/myproj-trunk
svn update
svn merge --reintegrate http://myserver.com/repos/myproj/branches/myproj-branch
svn commit -m "Merge myproj-branch back into trunk!"

can also test before merging with --dry-run
merge status code:

A  Added
D  Deleted
U  Updated
C  Conflict
G  Merged
E  Existed
R  Replaced
I  Ignored
M  Modifed
X  Unversioned, used external
?  Unversioned
# Missing 
~  Obstructed by another item

Delete my branch:

svn delete http://myserver.com/repos/myproj/branches/myproj-branch -m "Remove myproj-branch."

To see which changes have been merged to which branches:

cd myproj-branch
svn propget svn:mergeinfo
svn mergeinfo <source> <target>  #pwd if no target

#Which changes have already been merged from trunk to branch

svn mergeinfo http://myserver.com/repos/myproj/trunk

Which changes are still eligible to merge from trunk to branch:

svn mergeinfo http://myserver.com/repos/myproj/trunk --show-revs eligible

Show changes since creation of branch:

svn log --verbose --stop-on-copy http://myserver.com/repos/myproj/branches/myproj-branch

Merge 2 branches from same parent:

cd myproj-branch1
svn update #get what is latest revision
svn log --verbose --stop-on-copy http://myserver.com/repos/myproj/branches/myproj-branch2   # at wich revision was branch2 created?
svn merge -r 10:15 http://myserver.com/repos/myproj/branches/myproj-branch2   #merge modifs since branch2 was created to branch1