I'm just going to throw this out there because there are always new people who need this information. If you're going to lift even remotely heavy on movements such as bench press, incline bench, decline bench, reverse grip bench, and you want to go to failure ( of course you want to go to failure ), then you need a spotter. I happen to find myself in a recovery week this week with a strained bicep / shoulder because I failed to follow the advice that I am about to give, and as a result, I chose a terrible spotter. I'll break this down into to useful categories; tips for picking a spotter, and tips for not being a terrible spotter.

TIPS FOR PICKING A SPOTTER

Be polite when asking. Not only have I never turned down a polite request for a spot, but I honestly don't believe I have ever heard anyone turn down a polite request for a spot in a gym.

COMMUNICATE! This gets broken down a bit further. First, make sure that they are good spotting you at that weight. This is one of the mistakes I made that has led to my aforementioned recovery week. Don't assume that someone can handle spotting you just because they look buff or because you've chatted with them and they seem cool. Ask if that weight is ok for them.

Tell them how you want to be spotted. I always end up being the designated spotter in every gym I'm at simply because I actually ask the person how they want to be spotted. Most people won't ask, and as a result every new spotter is a roll of the dice. For instance, I find it really distracting when someone does that thing where they spot you by having their hands sort of wrapped around the bar without actually touching it. I nope that stuff all day. Other people like that. I don't even want to see your hands. I have never completely bailed mid-lift. Plus, I hitch about halfway up but am usually able to push through it. This is why I make a point of telling everyone who spots me that I am good unless I say "take it". I make a point of stressing this. Maybe you prefer seeing those hands there, hovering around or just below the bar. If so, let your spotter know.

Give your spotter an honest expectation of what you're about to attempt. If you're thinking three reps max, don't say five to look cool and then try to act surprised when you gas out after one and a half. They need to know what you're thinking because 90% of heavy lifting is mental. If you're really thinking three is your max at that weight then they need to be ready when you start that second rep.

Thank them.

TIPS FOR SPOTTERS

Do not agree to spot someone unless you are certain you can take that weight from their chest to the rack on your own. If someone is up in the mega weights they may need two spotters positioned on the sides rather than one behind the rack. Either way, if you can't bail them out of they blow out a shoulder or elbow, don't put them at risk by agreeing to spot them. Also, be up front about why you are declining.

COMMUNICATE!

Ask how they want to be spotted. Find out where they want your hands etc. Ask if they want their first fail rep to be their last rep, of if they want to go one past fail with your help.

Make sure you know what they are expecting to do, and then cut that in half in your mind. If they say 5 reps, be ready after the second one.

Be a cheerleader. You can actually talk someone through that last rep, or through another few inches of that last rep, if you take the right approach. Let them know you're there if it starts going down, but encourage them to push, to lock it out.

Give as little help as you possibly can if they are still pushing. Once you see them give up on the rep, then you need to take control of the weight. But as long as they are fighting for that rep, help them fight. Give them the least amount of help possible from the to succeed.

Don't panic when someone starts to fail.

NEVER break the plane of the movement unless you have full and complete control of the bar. I cannot stress this one enough. It's such a common sense thing, but it's also the most common spotting mistake. If someone starts to fail and you grab that bar, you pull straight up until you are clear of the rack and then you pull toward the rack. Again - straight up until you are clear of the rack. Any bench press is a delicate balance of chest, shoulder and tricep. They were pushing whatever weight is on that bar based on their experience using whatever balance they prefer given hand spacing and shoulder slot. If you pull the bar toward the rack rather than before you've cleared the thumbs on the rack you are pulling the bar and their hands higher than their shoulders and effectively removing their shoulders from the lift. In short, all of the weight is now on you. When you pull up they are able to help you help them right up to the point you get the bar on the rack.

That's all I've got for now. Please feel free to add more in comments.