>One of the problems we have had in the past with many projects is "feature creep". We have been using a fixed bid system with a set number of deliverables which protects us from large losses. After one deliverable, if we don't get paid, we stop working on that project. There is a fine line between what is an acceptable "enhancement request" that is unbillable and one that we would have to bill for.

IMO, if you're using a fixed bid system with well defined deliverables, no enhancement request is 'unbillable'. Reasonable enhancement request, fine, but your initial contract should state that any additional requirements will mean additional cost, i.e. it is billable.

>We don't want to seem to inflexible.

Flexible makes sense, but working for free doesn't. Flexible means you realize the project specs will change over the course of development. But you state this in your original contract and make it clear that increased features will cost more. This doesn't make you seem inflexible or greedy, makes you seem intelligent, experienced and realistic that you anticipate this.

Whil Hentzen (www.hentzenwerke.com) has some excellent references on how to spec a project. Definitely a voice of experience.