Words of wisdom. If you aren't really able to walk away from a job, you're not in the drivers seat.
In your case, I'd recommend trying to work 6 month contracts and always try to have a new one in the queue for more money, if possible, so you can ask for an increase at the end, or take the other job. But be classy about it.
The real fun part is when you're so over-flowing that you're hiring full-time engineers and you've turned into a consulting company. That's what happened with my last company.. I don't recommend it though, consulting isn't very scaleable, and there are no big exits.
In your case, I'd recommend trying to work 6 month contracts and always try to have a new one in the queue for more money, if possible, so you can ask for an increase at the end, or take the other job. But be classy about it.