As someone who's had one failed SaaS and one that is on track to be a success, the biggest takeaways have been:

1. Make sure there is significant a market for your service 2. Get funding 3. Expect things to take twice as long and cost twice as much 4. Talk to your customers constantly and don't be afraid to ask them to pay 5. Go back to point 1 and double, triple, quadruple check there is a significant market for your service