I'll divide these observations into those relevant for each phase of getting from a new business idea through about the second or third year of operations. After that (if you are the unusual start-up that makes it that far), things become quite different again. Caveat emptor.
IN THE BEGINNING..
You are selling a dream to prospective investors, employees and customers. Be ruthlessly honest with yourself at all times.
Have a co-founder. You're getting married to this person, so make sure you trust him or her, have great mutual respect, and can speak openly at all times. Drama is your enemy. Ideally, you should have high overlap in your view of the world, and only partial overlap in your skill sets.
Seek blue water. Do something innovative enough that nobody else is even trying. This is the best way to get around scale advantages that others have over you. Since you don't know what you're doing yet, it's better if nobody else does either.
Ignore capital markets and industry analysts until you are ready to exploit them; they are rear-view mirrors. If you are doing at start-up phase what the capital markets say is valuable, you're usually too late. You have to do what they say is stupid now (or are just not thinking about now).
In several years, you will either be proven wrong (in which case, they will say "told you so"), or you will be proven right (in which case, they will shamelessly ignore their prior opinions). It's nothing personal; it's just their business model.
Plans are basically useless, but a small amount of planning before you start can be valuable. A good business plan is done in Excel, not Word:
-- See if you can (A) make a list of the companies that might buy your offer, (B) estimate what each would pay for it. Multiply A by B. This, not "the CRM market" or whatever, is how to think about market size.
-- Guess at how many people at what comp level by person you would need to provide some early version of this offer. Gross up the headcount by a simple factor for rent and other costs, plus any other very expensive capital equipment you need to make a simple cash flow statement.
-- Figure two years of this cost run rate with no revenue, then some time with revenue at a loss, to get to break-even. This cumulative total is your capital requirement. Double it. This is still probably an under-estimate.
-- See if it looks like revenue might be greater than cost at some kind of a rough, early steady-state.
-- Always assume an average amount of luck in the long-run, and terrible luck in the short-run.
Fund it yourself for as long as you can stand it. Use your own money. Get some kind of revenue coming in the door to offset costs partially, but at least as important, to demonstrate that this is a business, not something on a cocktail napkin. However, don't make the mistake of becoming a consultant doing fee-for-service work, and not really become a technology company that can scale.