If you hit cmd-R to reply to one of your own tweets that was in turn a reply to someone else, your new reply should automatically be redirected to original reply recipient. For example, if I hit cmd-R on my own tweet "@someperson You're a cad!", the resultant tweet shouldn't be "@myusername ...", but "@someperson ..."

Twitteriffic has this feature, and I've come to rely on it, especially since you can add multiple replies to a tweet, in quick succession, and also especially because the original tweet might be far down the timeline.