I have done this as well. In fact, my recommendation to Alan Weiss is also a form of fixed price quoting. Granted Alan's fees are high, but still fixed. Scope creep is a problem regardless of how you price unless it is by the hour.
I believe the secret is clearly defining what is included for the price. Then include a simple statement that says items outside the scope are subject to another project fee or fees above and beyond quote.
Now when scope creep occurs, and it will, you need to make a few decisions. First, is the request small enough that by doing it you aren't really out much, but it could be used to give a lot of goodwill to a client that is already paying a substantial fee and may come back in the future? If so, I usually just do it.
However if I have a low or late paying client and the request is larger than a simple change or I doubt I will get any future work from them then I simply tell them straight up that their request is outside of the scope and it will be "X" to add it in there. For the low/late payers this is usually enough to back them off. If they really want it they will pay for it. If on the other hand they were just trying to get extra work added and figured you wouldn't object then doing this tells them anything outside of our agreement is going to cost them more. Note you may have to repeat this process 2-3 times for the stubborn client, but eventually they will get the lesson.
By no means am I perfect at this. I think it is always a learning curve, but I am really starting to see that scope creep from small clients with little chance of repeat business can kill you and you have to be tough and show some spine to keep from losing money.