Only the Strongest Survive
New York City housing market is increasingly difficult for would-be renters, with one national background-checking service alone rating 41 percent of applicants as either 'reject' or 'maybe'; city's annual income threshold of 40 to 45 times monthly rent, far higher than national average of 36 percent, is major stumbling block, often forcing potential tenants to offer extra cash upfront or guarantors; less than sterling credit history and any record of involvement in landlord-tenant court case are near guarantees of rejection; advice on researching credit history; drawing; photos (M)

