If you have access to a truing stand that's sturdy enough, and anchored to something that's heavy enough so that you can give the rim a decent sideways pull while tightening the spoke, then you can get the proper tension.

Another way would of course be if you have an endlessly patient helper, then one could do the "spoke untwist" thingy with pressing the wheel down against the axle while the other is tightening the spoke.

He used to post at cyclingforums.com, but that site has been badly hit by spam recently. Don't expect too much though, he can be quite testy at times.