Resting generally involves 'shaking out', where each arm is alternately released from a good hold and shaken to help remove lactic acid build-up. For best results, first shake your pumped-up forearm above your head to drain the lactate, and then below your head to flush in fresh blood. If you don't shake it above your head, the lactic acid remains trapped in your arm.
With practice a whole range of cunning rests can be used. These usually involve stuffing part of your body into holes and grooves
The key to efficient resting is to remove as much weight as possible from your weighted arm. Use your feet to pull your body close into the rock. Look around for better rests while you are shaking out your arm - a subtle change in body position can make a big difference. Generally the best rests involve large features. Look for them before you leave the floor. The most obvious rest place is a corner, where bridging out might even allow both hands to be removed.
With practice a whole range of cunning rests can be used. These usually involve stuffing part of your body into holes and grooves. Knee bars, arm bars, bat hangs, leg clamps and even head jams are all commonplace although often these may be difficult to actually get into. For example, your arm might not fit into a hole for an arm bar. Make a quick decision; don't waste energy, after all you are supposed to be resting.
Between attempts it is hard to specify exactly how long to rest and what exactly to do. In most cases you will spend some time belaying. It is crucial that you do not get cold - wear everything you need. If your mate blows his redpoint at the first bolt then spend your time moving around. This will maintain your blood flow and promote quicker recovery. Walk up and down the crag to look at other routes or if you are keen, do some stretching.
You do not get stronger when you climb. You get stronger when you recover
Rest for as long as you need, though this may be difficult to gauge. How you finished your last route will have a great effect, if you fell off and were completely trashed, you may need over an hour to recover. Generally, for pure power routes, around 20 to 30 minutes is enough, though for power endurance routes you should allow much longer. Try not to cool off too much. If you rest for an hour or more, you will require some kind of re-warm up, even if it is just a light jog. Time your rests with a watch if you need to.
You do not get stronger when you climb. You get stronger when you recover. So make sure that you allow yourself enough rest days! Lack of rest will not get you to the belay faster and ultimately it can lead to injury. How much rest you need depends your individual response to the activity and in particular, the intensity of effort used. A lot of pure power climbing often requires two days recovery, whereas stamina routes may not force any rest days.
You could stretch or cross train on a rest day
However, resting is often dictated by where you are. On a holiday you may want to get in as much climbing as possible, but there's no point in losing the last few days of climbing if you did too much too soon and didn't pace yourself over the first few days. You can often incorporate different styles of climbing in order to maximize your time on the rocks. Vary your climbing, if your fingers are sore and fatigued, perhaps a second day on could be on steep 'juggy' rock.
For those with spare time, 'active rest' is the best way to use rest days. Stay active - resting the principle muscles and joints that you use in climbing by walking, jogging or doing some other sport to aid the recovery process - and increase your overall fitness (but don't overdo it!) Easy climbing or an alternative style such as scrambling are ideal forms of active rest for sport climbing, as you are constantly learning more about movement and having fun at the same time.