While BasicallyMoney.com offers a good consumer's perspective, keep in mind that you're in the business to make money. Everything single thing that you can possibly do (even, say, give away free $100 bills on the street) will turn people off.
Don't be afraid of turning people off. That's called qualifying, and it's a critical process to making sales. But the difficulty for you is that the answer to every single question you're asking is "who knows, it depends."
1) Length - 15 or 30 days?
How long will it take to evaluate the service? Hoover's offers a twenty-four hour trial. Microsoft Servers are often 120 days.
2) Do we get the customer's credit card info at signing up or after the trial, to reduce friction at signup?
Someone who gives their credit card number is a much more series prospect than someone who just fills out a form. Who do you want in your sales pipeline? What's the conversion rate difference?
3) Is the trial supposed to be free? Or do we charge for it if the customer doesn't cancel?
It doesn't have to be. For services that need longer commitments (6 - 12+ mos), it may make sense to allow people to try it out for just a single month at 1/12 the cost of a year subscription.
3) Do we charge when the trial starts or when it ends?
If you're going to charge for a trial, I'd charge at the start. If the service was a poor fit, it's better to have the payment for that service be as far away from the realization of the poor fit as possible.
Basically, there's no way of knowing the answers to these questions until you try them out.