Ret=1

Appending ?ret=1 to a Login URL, will return you to the page you were on after you log in. It is possible to specify a different URL address to return to as well.

http://www.livejournal.com/login.bml?ret=1 http://www.livejournal.com/login.bml?ret=http://pics.livejournal.com/manage/

There are limitations to this however -- LiveJournal only redirects to other pages on LiveJournal, or other hostnames that it trusts, like pics.livejournal.com.