Cash flow. A high proportion of small businesses which are theoretically viable fail because they run out of cash and can't pay suppliers (or the mortgage). This means invoicing soon after the end of the job and chasing up late payers. For large jobs, stage payments.
The other thing to consider when pricing and estimating completion is that far less than 100% of your time is spent doing the actual making part of the job. Design, accounting, invoicing, marketing, discussing with potential clients, ordering and collecting supplies - all these have to be funded, along with equipment, premises if you have them, training if you need it, etc.
And finally (speaking as a lawyer by trade) you want a clear, agreed statement about the scope of the work. Changes cost and need to be charged for, so explain the estimated cost of changes in advance and get agreement to that. In the work I've done on procuring high value computer systems the big arguments were always about this, agreed in an informal discussion and never written down. I wouldn't get anal about formal contracts though - a simple exchange of emails can be contract enough if it's clear. In essence, what you're going to do, for how much, by when, and when you must be paid.
Good luck, 3 months work in hand is a good start.
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