One of the main problems when it comes to make a timelapse is to have everything well exposed, and even if shooting everything in RAW helps, and shooting in Aperture Priority, letting the camera adjust itself it might help as well (even if I cannot see myself set my camera to AV, I'm an extremist, and for me only M and Bulb exist), it's not bulletproof, light condtions changes, a small cloud in front of the sun can drastically change the exposure from the previous shots, just to mention one of the possible problems. But there is one technique that it might be the answer for most of the problems and it makes timelapses look and easy job. The technique we are talking about is the Bulb Ramping.
In the video above, Joel Schat, shows us how it works, and in the video below, there is a clear example of what the results are using this technique.

(via FStoppers via Joel Schat)