How do you convert existing trial users to paying customers

I'm running a B2B SaaS. Potential customers can use a trial version as long as they need, because it's a rather complicated white label solution and most customers need time to be convinced that they should invest their time on it.

I find it difficult to get people to start paying. It's probably not that they don't want to pay, because it's great software and most potential customers. I talk to them on skype and somehow I find it difficult to find the right moment to ask about starting billing.

Maybe I can send out an email on a weekly base to ask if they're ready yet or need more time using the trial version. But maybe you guys have a better solution.