Start with your passion.
If you pursue something that is close to your heart, you are more likely to succeed - and you'll never have to worry about losing interest.
Do your research.
You've got the great idea, but will it work? Road-test it if you can before you chuck in your present job. But don't over-research, either - otherwise you'll end up never doing it.
It's wise to talk to others who have chosen the same path. Find out what they like about working for themselves, but also what they don't enjoy. It's unlikely to be all positive.
Planning is everything.
In order to apply for business loans or other kinds of funding, you must develop a solid business plan. Writing one takes time, but no-one will give you money on the basis of a plan that has not been well thought out.
Know no fear.
Setting up a business is not a 100-metre dash - it's a marathon. You will be met by a lot of challenges and disappointments along the way. Don't worry: this is normal.
Sleep on it.
Are you sure you're ready to take the plunge? Not everyone is cut out to work for themselves.
Remember why you're doing it.
Whatever you achieve, you'll be doing it for yourself, rather than for someone else.