This way is used increasingly and enjoyed by many photographers in the world. Most of them often do some times to experience the best exposure. No advice is perfect because it also depends on how to shoot and intentions of each photographer in different contexts. But it would have to rely on some basic principles to have fine-seen works at least.
In this article, Pankha Nguyen would like to share the experience of his long exposure photography. It's not advice, just for reference only.
_ Setting up the main object in suit frame before making a shot.
_ Light and aperture are inversely proportional to each other. This means if the light at survey area is higher, the smaller the aperture is opened.
_ I usually do with the aperture from 11 to 22, the time at least 20 seconds and the maximum is 60 seconds at night. I like to do with a moderate amount of time.
_ I rarely make a long exposure shot before the dawn, in the low-light. But I tried several and often do with the aperture 8-11, and the time is very short, about 8-15 seconds.
_ I also experiment this method in the day-time, evenings exactly, from 5-5:30pm. The aperture is 11-22 and the time is 8-15 seconds.
See more of his work at his photostream on Flickr.