I've found the best way to sustain sales in general is to have a "Sale" whenever things are slowing down. I've been doing it regularly since I first started submitting apps - basically something like this at the top of the app description:
*** XYZ IS HALF-PRICE UNTIL THE END OF JUNE - REGULAR PRICE $7.99US - DON'T MISS OUT!***
This tends to boost sales for a while (everyone loves a bargain) and also helps to generate some additional reviews for the app (via Appirater code) which help once I put the app back to full price.
Unfortunately, it doesn't help when your app is priced at $0.99

Just another reason that apps should NEVER be posted at such a low price. Crazy App Store!