I refer my clients to a mortgage specialist with TD in Edmonton. She is able to work magic.
I believe there are two sources for her superpowers: 1) she is a mobile mortgage specialist working out of her home, so she only works on mortgages and doesn't have usual distractions one would find at a bank (like other customers); and 2) banks have leeway to make exceptions for clients, and bank staff can push for those exceptions, whereas brokers must fit everything into the box provided to them by the broker channel.
I have no doubt that there are some brokers that specialize in getting tricky cases approved, but I think having a banker on your side is helpful.
As for credit improvement strategies, one key is to keep balances on revolving credit lines and credit cards below 30% of the limit. If someone has a small limit (such as $500 or $1000) and must make a purchase of several hundred dollars, he should prepay the amount so that the % stays low. Even if people pay their balance in full every month, if their credit is pulled the day before the payment is processed, the score will be substantially lower.