Over the years, I’ve made several comments to people asking for bouldering and spotting tips. Since this subvoat is still growing, I figured I’d compile my previous comments into a list on how to boulder outdoors safely, and on how to spot.
First: what to know when you start bouldering outdoors.
Commit to the climb before you start.
So, look at the boulder. Is it dangerous? What would happen if you fell? Are your crash pad and spotter enough to keep you safe? If not, are you willing to accept the risk? If the answer is yes, decide that you're going to do it and focus only on climbing.
Your chances of getting to the top safely are higher when you can put your focus on climbing alone instead of on being afraid.
Don't be afraid to NOT climb stuff, however. I, personally, hardly ever do highballs, because I'm not willing to accept the outcome if I fall badly. I want the worst case scenario to be that I break my ankle(s), not that I break my spine. Bouldering is my great passion, but it's not worth my life.
That's just me, and as long as you're only responsible for yourself (i.e. you have no kids and your partner is on board) you can go free solo The Nose as far as I’m concerned, but take the decision beforehand.
This is important: Please don't so something just because there are others who 'are not chicken'. Do it because you want to climb the line and are willing to accept the risk. Get your mat in place, and a GOOD spotter. Then, once you're on the boulder, forget about the risk and concentrate only on the movements.
When pushing your limits, having a GOOD spotter is important. This means that you need to be careful choosing your spotter(s), but also that you need to take in account that if you accept to spot for someone, you can't just walk away/lose focus etc. Even professional climbers have had spotters walk away from them because their traverse took too long, and it can leave someone in a precarious situation. Think about it before you do it or ask it of someone.
Spotting is not something to do half-heartedly, though most bouldering videos will show you people giving beta, gesturing, paying attention to other things, etc. This is fine when you just want to have some fun with your mates, but if you're doing something that you might get injured falling out of, make the rules clear. And if you don't trust your spotter, it's better not to have one, because you need to be able to focus on what you're doing, not what they are doing.
Decide if you’re going to try the problem. If you want to do it, only focus on how to maximise you chances of finishing it, and on limiting your risks.
Now, check where you will climb/jump off the rock before climbing on it. Look for foot holds you might not be able to see from on the rock. Choose the easiest way down. You may be unnerved after finishing your climb, or pumped with adrenaline, so be aware that getting off the rock may feel scary too.
Don't jump off a rock unless you have to (i.e. - you can't climb down safely), because it's bad for your knees and ankles.
Then, look at the route you'll be taking and the moves you'll be making when deciding where to put your crash pad(s). Often people misjudge how far they will fly off the rock when falling. Think about whether crash pads need to be moved during your climb and whether you can take a rest at that point or will have to be less protected for a moment.
Talk these things through with your spotter. Find out if they are willing to commit to spotting you seriously all the way through the problem. Discuss whether you prefer getting beta (tips) or are taking risks and need them to be focused on helping you land in case of a fall only.
Put on your shoes, stand on a small mat in front of the start, wipe them clean (or even better: use your brush!), focus on the climb only, and go!
Now we come to the spotter. Some information on how to spot:
Before your climber starts climbing, take a look at the problem together. Will he/she swing out in unexpected ways? Where do you want to avoid having him/her land? How should you position yourself so you can spot them the best?
When spotting someone, fix your attention on their sides (when spotting a man) or hips (when spotting a woman). Don't pay attention to flailing arms or legs, that happens, but when a person's core is coming at you, they're probably falling and need spotting.
Keep your hands ready to take someone's sides/hips, so you are actually in time when they fall. It happens quickly, and unexpectedly at times.
Spotting, when you're in really dangerous situations (uneven terrain, surrounded by other rocks or trees, etc), is about one thing only: getting someone down feet first, preferably on the crash pad. This means you may have to push them. The point is to avoid them injuring their head and spine. If the fall's not that bad, try to steer them away from uneven surfaces to avoid sprained/broken ankles.
You can't really spot and give beta at the same time, because giving beta involves looking at holds/hands/feet, and that means taking your attention away from their sides/hips. If you're really spotting someone, don't give in to their questions for beta, because if their foot slips at that moment (or a piece of rock breaks off), you won't be able to catch them.
You also can't spot and move a crash pad at the same time. Communicate with your climber about whether you can move the crash pad. They should not continue climbing during that time.
As spotter, you are responsible for someone’s safety, and in some cases even the climber’s life. Just as when belaying someone, you should take this seriously. That means doing the utmost to ensure your climber gets to the top (and afterwards the ground) safely. However, some climbs are long, and it can get boring. You may at some point want to quit spotting. If you ever do, please tell your climber before you walk away, and only do so if they can get back down safely. But realise that you may well lose a climbing buddy.
Spotting tl;dr: take inventory of the dangers before your climber starts. Keep your attention focused on your climber's core, and hands at the ready. Push your climber to avoid them hitting their head or spine. Don't do other things while spotting. Don't walk away before the climber's done.