Don't you agree that you should be paid for that? Otherwise, you're just giving away free advice with no guarantee you'll be chosen to execute the project.
If you've done that one too many times, I have some possible solutions for you.
For example, I had lunch today with a client, Eric Koby of Fat Rabbit Creative, who was a freelancer until recently, when he decided to grow by hiring a couple of employees so he could bring in business. He's now finding it more "creative" to grow the business than to do the design.
Anyway, he described to me his new strategy for not losing money scoping web projects. He charges a simple "meeting fee" to sit with a client and help them figure out what they need. It could be as easy as that.
Another more involved (and more lucrative) option is described in an article I wrote for Applied Arts last month. It's called, "An Alternative to Writing a Proposal" and in it I interview Erin Pheil of Timeforcake Creative Media about her Web Site Blueprint service -- essentially the discovery phase of a project broken out as an a la carte service and offered for a flat fee.
And if you want to know more about exactly how to do this for yourself, join me for a webinar on Sept 12 for the Registered Graphic Designers of Ontario in which Erin will show an actual example of the Web Site Blueprint and share details on how she sells it to her clients. Should be very enlightening!