I went 500 down at one stage too - went on holiday for 2 weeks, pile grew, motivation decreased, pile grew, and so on up to 500 to review.
For quite a while I left it that way because I didn't realise how easy it could be to get back to 0.
Tips for people trying to claw back:
* If you have a graph of kanji added over time (I find graphs great for motivation), add a second line which is (total kanji in system - kanji to review). Watching this line rise will provide good motivation.
* Review box 4 first.
- You've reviewed them 4 times already, so retention should be good.
- Once you've marked them as remembered, they won't be back for a long while.
- This will quickly bring down your "to review" number.
* Work backwards from box 4, and let failed kanji pile up in the first box (ignore them until the end).
This way, you make quick progress purely by doing reviews, and you don't spend time languishing in the study section while your "to review" number is massive.
Lastly, once your "to review" number has been severely beaten down, you are left with a fair-sized fail pile. Hopefully, the motivation from getting this far quickly will help you work through these, say, 10 at a time.
Hope that's of help to someone, certainly helped me get back in the swing of things.