Finish client projects as quickly as possible. The longer a project takes, the more money it costs (and if you are working for a flat fee, you will be the one paying), there is a greater chance that the client will become unhappy, and it can contribute to the feast-famine cycle of freelancing.
Consider:
Starting and stopping a project takes additional time. If progress stalls, it will take time to reorient yourself as to where you left off. This extra time adds up over the course of a project, and can have a big impact on how much money you make. Of course if you are working strictly by the hour, this may not be so bad for you, but if you work on a flat fee system, the financial hit can be substantial.
The longer the contract is open, the greater the chance the client will try to slip in additional features. This can be a good thing (more features should equal more money), but it is important to have a solid contract and scope document. In my experience, clients are really good about finding gray areas in the scope document. To me, it's preferable to get their project done and move onto the next.
The longer it takes, the longer it takes. As a project drags along, both parties can become disinterested, which takes away motivation from actually finishing, which creates a vicious cycle of delays. It is in your best interest to ensure that a project doesn't get to the “complacent” stage.
There is no motivation in working for free. If you are working at a flat fee, there comes a certain point where you are working for free (I say free, because I find it easiest to stick to a minimum hourly rate, where any hours that I exceed my time allotment for the project are pro bono. This makes it easier to track how much money I make per week, since I don't have to guess at what my hourly rate actually will be). Working for free is no fun, and since I still have bills to pay, I have to balance my unfortunate pro bono time, with actual billable time. This means that the overdue project falls to the bottom of my priority list, which certainly doesn't contribute to getting the project done any sooner, and so begins another cycle of delays.
The longer a project takes, the longer it takes to get paid. This can have pretty substantial impacts on meeting weekly/monthly financial goals, and if you don't have monetary reserves, can cause you to use credit cards or other loans to fill the gap. This in turn, costs you even more money since you will now be paying interest on your loan.
