In my observation one common stumbling point for climbers of a surprisingly wide base of experience is that of safely and efficiently setting and cleaning sport-climbs. In theory the process is easy but the reality of the situation is that many climbers unintentionally expose themselves or their partners to large degrees of unnecessary risk, time after time, without realizing the magnitude of the risk they are exposing themselves to.
I will not go into the nitty-gritty of a step by step process of what to do and when as routes and preferences vary, but rather will outline a few key rules which can be shot for and a few common problem situations to try to avoid.
If lowering off of a climb make sure your belayer has a blocking knot tied in the opposite end of the rope and that they are paying attention to the rope length closely to prevent lowering the climber off the end of the rope or creating an overall poor situation (common cause of accidents).
Due to the fact that a leader generally leaves their rope clipped with draws to the wall below him/her it is generally acceptable for them to clip two draws at the anchor bolts with non-locking gates apposing and lower off them or clean the anchor and lower directly off it.
If a climber wants to subsequently top-rope a pitch after it is lead, however, the rope should be attached to the anchor with locking carabineers as the top-roped climber will be solely relying on the anchor when they reach the top of the climb after unclipping from all the intermediate draws on their way up.
When cleaning an anchor (and most importantly a top-rope anchor) keep the rope clipped into the anchor before threading the rope through the chains to reduce the potential of taking a large fall (to the ground in the case of a top-rope).
Stay on belay through the transition process. If you need slack in the rope ask your belayer for it, but never ask to come off belay if you are setting up to be lowered. This is another very common cause of accidents.
Before untying and threading the end of the rope get in the habit of clipping off a bight of rope to your belay loop with a figure eight on a bight on a locking carabineer to prevent from dropping the rope and not clipping off the rope to a non-load bearing portion of your harness, like a gear loop. This way if your balance or your attachment to the anchor fails for some reason your belayer will still have you in the system on belay.
If the anchor chain links are gauged wide enough try threading a bight of the climbing rope straight through the anchor and clipping the bight with a figure eight on a bight with a locking carabineer to your belay loop prior to untying your original knot. This way not only are you are never actually untied fully for any length of time but the anchor is threaded in the process. Note that by using this method a couple meters of rope tail will be present off the end of your new knot. It is imperative to ensure you have enough rope to still be lowered off. Lower with the carabineer attachment.
Generally speaking, if you choose to clip to or weight an anchor while cleaning it consider whether you will be attached to the system in other ways additionally (such as with the climbing rope below or not) to determine whether you need to be attached to a true anchor point to hold your position or rather just a point to hold your balance. If the anchor is your only point of connection to the system be sure to satisfy all the rules you normally would with any anchor point. It is all too common I see climbers clean anchors with a P.A.S. tether clipped to one bolt as a sole connection to the system. Don’t be that guy!
If you would like to practice these skills in the field please let us know!