One of the best ways to spread your risks if your income is dependen on web directory is to have more than one web directory.
You should position your secondary directories differently to your main one and also target different keywords, this way should you lose groungs in SERP with one your secondary directory will still bring in income.
You should position your secondary directories differently to your main one and also target different keywords, this way should you lose groungs in SERP with one your secondary directory will still bring in income.