This is where the "star trail" technique comes in, another thing you really want to automate—the first few timestacks, I made one photo at a time and it took forever. I couldn't figure out how to properly automate the stacking process in Photoshop, so I searched online and found a script from Star Circle Academy (www.starcircleacademy.com) that worked great.
Basically, it opens the first image, then pastes the second image on a new layer and changes the Layer Blending mode to Lighten. The script adds all the parts of the second photo that are lighter than the first photo. Repeat that as many times as you like with your succession of photos.
Sometimes, I'll use only 30 photos, and other times I'll use hundreds; it's all about the desired effect and what looks best to me, so I usually start out by stacking all the photos from the time-lapse and then try fewer photos if it's too crazy.
Once I've found the segment that looks good, I do a final adjustment of levels and contrast, mainly to bring the darks back, because it gets a little washed out from adding lighter parts together.
The camera I'm currently using is a Canon EOS 60D (I've gone through more than a few). I use a battery grip that lets me use two batteries so I can shoot longer time-lapses. I most often use my wide-angle Tamron 10-24mm zoom, but I also have a telephoto and a 50mm prime lens. I have an assortment of filters; a circular polarizer and a dark neutral-density are my favorites.
Most Canon cameras can utilize open- source software that allows the camera to do things it can't straight out of the box. I use a program called Magic Lantern that allows me to do all kinds of fun new things, but the main reason I got it is for its intervalometer, so I can easily shoot time-lapses. You can get it at www.magiclantern.fm. All you have to do is put a small file onto your memory card and it loads from there with new menus. Another bonus is that there's no external hardware involved. Before that I was using a separate intervalometer and going through lots of batteries!
A tripod is more or less essential for shooting time-lapses. I also recommend anchoring the tripod; that will keep it stable and safer to use when you're outdoors. My camera is in for repairs right now because I failed to do this on a windy day. I've found that a brick on a rope works well. Make sure to hang it close to the center, so it's distributing the weight evenly between all three legs.
If you like the results you see here, why not try a few yourself? If you need help, feel free to contact me—you'll find contact links and also can see more of my work at www.flickr.com/photos/matt_molloy/sets/72157631635631443/.