I mainly use lightroom, but I'll take a bit more care to get the details correct particularly when it comes to sharpening and NR sliders. Then I'll soft proof with the labs ICC colour profiles and output a jpeg at the size and ppi I want, do a final check and upload. The Lab I use has a program with all their products in it so I use that to specify the options then upload and order from there.
I have noticed that the Lab I use do up to 5 free printed samples to help you proof, which I'm planning to do soon.