Obviously the more I try and memorize, the more the first things I started memorizing tend to fade. This is a common thing we humans deal with on a day to day basis. On average, the human can hold about 7 things in the short term memory, and the more recent something is, the fresher it is in your memory. To actually transfer information to the long term memory, one needs to review and repeat the information.
What about speed memorizing? So when I first started getting good at memorizing numbers in 5 minutes, I would go through about 180-200 digits with only looking at them once. I used to think that that was the crowning achievement of memory techniques, that with one look, you had it in your brain! I still believe this, but, as I tried to gain speed and more digits, I found that it became easier for my brain to forget the earlier digits. So then, after competing in the Cambridge Memory Championship earlier this year, the amazing Christian Schaeffer told me his technique. He flies through the numbers for the first 3 minutes, not really memorizing them (more like just watching a movie in fast forward), and then goes back and speed reviews again before the time is up. He seemed to do that for every discipline, whether it be names and faces, numbers, or random words.
I then found out that the #2 guy in the world in memory, Johannes Mallow, does the same thing. And he can nearly memorize 500 digits in 5 minutes! So I had to try it. I've been doing it for about 6 months now and the results are amazing. And whats crazy about it is that it never really feels like I'm memorizing or that the information is sticking. But somehow, it's always there when I go back for it! Amazing!
The point is that the memory techniques I preach are undeniably successful, but review, as an additional weapon, will make information stay in the brain and even allow you to cram more information in your brain as well. I almost never want to keep the numbers and other information I memorize in my brain, so I never consistently review them after I'm done. I force myself to forget. But, if I wanted to keep something in there, I could; no problem. It would just involve reviewing on a consistent basis (every day, then taper to every week, then to every month, etc), until it eventually moved over to long term.