What you want to accomplish is usually called cherrypicking in version control systems.
Say that you want to merge revisions 345, 364 and 377 from your branch to trunk, you will do the following at the top level directory of a clean working copy of trunk:
svn merge -c345,364,377 ^/_your_branch_
on Windows cmd add quotes around the branch name:
svn merge -c345,364,377 "^/_your_branch_"
You can find more information in the corresponding section of the SVN Book.