Entrepreneurs working in financial technology say everything from tough new rules to tweeting is shaking up the old-school world of banking. But while there’s opportunity, chasing big business on Wall Street can be tricky for tiny startups. Anand Sanwal says selling to the financial industry is exhausting....he’s the co-founder of CB Insights, a venture-capital database. Yaron Samid, founder of BillGuard, says the financial industry’s long sales cycle also makes it tough to iterate. But when you finally win a big customer’s business, the payoff is worth it, as once you’re in, you’re in. David Kedmey has found that as he sells his financial analytical software, EidoSearch, to large mutual funds and banks. All three fin-tech entrepreneurs provide advice for those thinking about launching a fin-tech startup: Know your bank contact; talk to your customers early; and be smart about seeking advice.