There is a lot to say about sequences, but the two most important principles to remember about sequences are:
1. They are the screenwriters best friend. They are small and self-contained enough that they can be kept in mind all at one time (unlike the entire feature, 90 to 120 pages), can be written in one burst of energy and can be shaped and honed independent of each other.
2. They must absolutely have a clearly defined tension in order to have shape. By shape I mean that the audience starts to take a vested interest in one or another turn of events in the near future (and therefore is involved, participating) and they have an idea of how things are going - hope versus fear. Once that overall shape is established, the sequence itself can contain all manner of desperate elements as long as this tension is periodically kept alive.