Most laser accidents occur while aligning an optical system. When you first place a mirror in a beam, you do not know where the reflected beam will go. If the vertical adjustment happens to be near the end of its range from the last time it was used, the beam could be sent careening upward - eventually to reach eye level. Fortunately, it is easy to align a laser system safely.
The first rule is to align using the lowest beam power you can. There will usually be times when high power beams must be aligned, but the coarse alignment must always be done at low powers. If you want to direct a beam into an area, where someone else is working, you must inform this person and agree amongst yourselves how to do this safely. It may be that one of you will have to wait until the other is done. (This is all obvious, of course. This document does not wish to suggest that safe practice requires much more than common sense. The goal of this document is to guarantee that everyone sees these ideas formally, at least once.)
The greatest danger occurs when a mirror is first placed in the beam. This danger is completely nullified if a beam block is placed in the reflected path before the mirror is placed. Since you are doing this coarse alignment with low powers, almost anything will serve as a block. Once the mirror is in place, you can carefully move the block to the location of the next optical component, always keeping the beam on the block.
You should follow the same procedure with any optical component. Lenses require a little extra care, however. Here you must also know where the reflections will go and how they are focused, before you place the lens. Also, note that a focusing reflection can easily damage any nearby optic. (If you notice a spark appearing just before or after a lens that is probably a reflected beam breaking down the air.)
The lasers that generate the beams are all at roughly waist height. You should strive to keep your beams at this level. This tends to keep spurious reflections in this plane where they are easy to block. In addition, everyone is careful to keep his or her eyes well above or below this level. However, there will certainly be times when you have to send a beam from one level to another. Just be aware that this is when you are most likely to create a reflected beam moving upward at some random direction. We have a large laboratory, and if this beam is not blocked it will reach eye level. This is simply not acceptable.
You will often have high power beams present that you no longer need. For example, you could send a 1064 nm beam through a nonlinear, doubling crystal to generate 532 nm light, after which you may no longer need the 1064 nm light. We have high power "beam eaters" that can swallow any (non-focused) beam generated in the laboratory. If none are available, they are easy to construct. Ask someone.
The hand-off. If you and another person are working on a project, one of you begins aligning a group of optics, but the job is finished later by the other. This can be hazardous. As you work, you become attuned to the specific problems at hand. The person who takes your place does not have this awareness. Make sure you leave your apparatus in a safe state and that you point out any problems you have noticed.