after being bitten and seeing some of my coworkers bitten one too many times, i sat down and wrote up an outline for a Requirements Specification. it's basically a list of questions that really need to be answered before development begins to make sure that the developers and the customers are all operating with the same assumptions. the Requirements Spec is in addition to a regular functional spec (the Joel article that guha mentions is excellent for that).

depending on the size of the project, i'll either insist on a full requirements specification being written, or, for smaller projects, just take a printout of that outline to the initial meetings and use it as a checklist to make sure everything that needs to be asked is.

don't skimp on at least gathering the requirements. i'm not convinced that they always need to be written up and signed off on but you definitely need to go and systematically make sure you're all on the same page. be precise and annoyingly thorough. i've had meetings with customers before and they say that a "small" class will be using the application. i imagine that that means around 20 - 40 students. in their mind a "small" class is 400 students. only an order of magnitude difference. not finding that out till the end of the project can be a painful experience.

anders pearson