I know this method's going to be a bit controversial, given all the emphasis in muscle magazines the past few years on giving your muscles enough time to "recuperate" and "repair." But, let me explain, and maybe I'll have a few converts (especially once you put the method to proper use).
I think it's mistakenly believed that bodybuilders of the past trained so frequently (usually 3x weekly for each bodypart) because they simply didn't know any better. But, if you were to ask the great Bill Pearl if he would change the way he used to train considering all the new "knowledge" about recovery, he would flatly tell you, "no." The same goes for longtime Iron Man contributor George Turner. He's seen it all, and done it all, and still believes frequent, volume-oriented training is better.
One of the reasons bodybuilders who train each bodypart once-per-week get so sore is because, well, they train everything once-per-week. This never allows you to increase your rate of recovery, because the demands are never placed on your body to do so. Sure, if you start training everything two, or even three, times a week you're going to be sore, but after a couple of weeks the soreness will subside. Then, look out, because it's growth time.
Strength coach Bill Starr has trained a lot of lifters over the years, and he still believes three-times-a-week training is best for each bodypart. Even if his lifters could get the same strength gains from once-a-week training that they get from 3x training, he wouldn't let them do it. Why? With once-a-week training an athlete simply never gets in good condition. Old time bodybuilders knew this.
Now, I'm not suggesting you rush to the gym and start performing the same "heavy," all-out workout you've been doing once-a-week and increase it to twice-per-week. Start off by adding an extra "light" workout 72 hours after your "heavy" session (no matter how sore you are). After about a month, increase the frequency to three times-per-week, using a heavy/light/medium rotation. The longer you train this way, the harder you will be able to start training at each session. And remember, it does take years of training to reach up to the type of regimen Bill Pearl used.