I've noticed a trend lately where clients are asking more questions and being more thorough when hiring a copywriter. They're not just throwing their money at projects. They're being more methodical while looking for a talented copywriter.
My guess is they've probably hired copywriters before without it paying off. And they want to make sure it doesn't happen again.
My tip for copywriters is to become at least proficient with multivariate and split testing software. Let them know you'll be paying attention to their conversion rate and running tests to improve it... maybe even as part of your main package.
This could go a long way to proving to the prospect that you're the copywriter to hire.
I typically use my own customized version of Muvar for multivariate testing for my clients. There's also Google Website Optimizer. And Mike Humphreys has his software as well: Easy Multi Tracking: Professional Copywriter Reveals His Proven Conversion Boosting Tool That You Can Use To Skyrocket Your Conversion Rates FAST!
There's also my free split test software: Free Split Test
Copywriting Dean|Copywriting And Internet Advertising Tactics By Copywriter Stephen Dean