If you can find a part-time job with relatively high wages per hour and some flexibility, that's a good compromise between working a full time day job and being completely unemployed.
Depending on your personal risk tolerance and risk attitude, you can supplement/replace your part-time income with savings or credit.
If you're really creative, you can do something akin to Airbnb's cereal stunt to raise money. :-p
Again, the answer to this question is going to vary depending on your situation, and even for the same situation, people will have different advice based on their own experiences and values.
However, during day job you can do a lot of market research and customer development and get to the point that you are confident that startup will succeed (or at least it will not fold immediately). Basically, find customers first (use demos and prototypes) then build a product.
And what I found out is that some people will actually stick with you as customers/users even if your product sucks - they believe in you. In other words, finding some normal people to believe in you and what you are doing is much easier that finding investors who believe in you.
I think there are a lot of variables involved, such as how much you value your current job vs. your passion and belief in the product you're personally trying to develop, as well as how much technical expertise you already have in the problem domain.
There may never come a time when I feel completely comfortable with the thought of losing the day job, but this makes the most sense to me right now.
Good luck!
If you think it's really that great, you should jump in, slash your expenses to the floor, and work 100 hours per week. Particularly if you have no family / kids to take care of; you can afford a greater level of risk.
If it's not that great, consider not doing it at all. Save your investment (time, money, heart, etc) for something better that will inevitably come along.
If you're not willing to suffer intensely, if necessary, for what you're building, it might not be the right thing to dedicate your time to. Sometimes it takes an extreme amount of sweat to get to the finish line, you have to factor that in if you're going to run the race.