If you have 1,000 words and you forget 8% of them, there will be 80 words you don't know! Relax, that's actually good for you. If you could set the target retention rate at 100%, you would waste a lot of time reviewing words you don't really need to review. The closer to 100% you come, the more time you need to increase each percentage point.
What do you do when you forget a character or word?
The upshot of the above discussion is that a healthy study strategy includes forgetting a relatively large number of words. However, I think it's a mistake to just treat this as part of the natural process of learning and just continue studying. It does matter what you do with the words you forget.
Actively dealing with forgotten characters and words
The most important word in this subheading is "actively". This means that you can't treat all characters and words equally, you need to adjust to each situation and base your action on many different factors.
Here are a few things you can do with a forgotten character or word:
Delete it. Do you need this word? If you probably don't, delete it. If you're not sure, delete it (it will surely appear again in your listening and reading, and you can add it again later if you want to). If you know you really need the word, keep reading this list.
Edit and upgrade the word. Actively look at the information you have and make it better. Create a new mnemonic (the last one obviously didn't work), add a custom definition or expand the current definition. Draw a picture. Spend some quality time with the word.
Drill down and learn more. Look up the individual characters in a word or the character components in a single character. Use another dictionary if the current definition doesn't suit you. If you're confusing two characters or words, look them up, you're unlikely to be the first to confuse them.
Add supporting characters and words. If you can't get a character to stick and it's an important one, add other words that also contain the character. If you keep forgetting how to write 凝, add several common words containing that character. You will see it a lot in the near future and you have a chance to really get to know it (and the new words, of course). You can add words from Skritter or use Hanzicraft to get a quick list of frequency-sorted vocabulary (for 凝 that would mean 凝固, 凝望, 凝聚 and 凝视).
The decision what to do with a forgotten word usually just takes a few seconds, but will lead to a much-improved retention rate without spending too much extra time. Actively trimming and adjusting your collection of words and characters also makes sure that you don't waste time on vocabulary you don't actually need.