Rumors, stories, bad information, and out-of-date procedures all contribute to confusion about how CPR should really be done. Here are some questions that folks have asked CPR Dude.
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How often should I stop CPR to check for signs of life?

Once you start CPR, you should not stop to check for a pulse or signs of life. While giving CPR, you should be looking for signs as you transfer from breaths to compressions, but you do not stop to check.

What about compression-only CPR?

See the first paragraph on CPR News.

How much of the heart's pumping does CPR really simulate?

About 25%, give or take a bit. So, yeah, you aren't being very efficient, but you're still pushing the blood around and that's the important thing. If you didn't do anything, a big fat 0% would get simulated, so 25% is a whole lot better than that.

I'm worried that I might further injure someone by moving them after an accident. What should I do?

First of all, you should only move someone if it is unsafe or impossible to treat them where they are. If you can do CPR on them, then leave them where they are. If you have to move someone, keep the victim's head and neck in its current position and supported as much as possible while moving. If the person needs CPR, and you do nothing, death is like 100% for sure. If you move him, you might cause further damage but that is a might versus a for sure.

Why don't you cover 2-person CPR on your site?

The ratios and procedures are the same for 2-person CPR as they are for 1-person. One person does compressions and the other does ventilations. You rest while the other is doing his job.
If you are the only one that knows how to do CPR, but there's a couple more people around, you can quickly teach them to do compressions if you are getting tired out.

How much good can it really do to breathe carbon dioxide into someone's lungs out of yours?

Normal air contains around 20% oxygen. A breath of air you exhale still contains over 15% oxygen so when it enters a victim's lungs, there is significant oxygen to be absorbed. The carbon dioxide mixed into your outgoing breath just comes right back out of the victim too.
Sure, they aren't getting as much oxygen as normal, but it's sure better than nothing.

How do you know if the CPR you're doing is working?

As long as you can see the chest rise and fall, you know air is entering the victim's lungs. But, you can't really tell if your chest compressions are moving the blood through the victim's body.
Spontaneous revival of a victim just from administering CPR is not common - but AEDs are now making it more probable. Your goal is to keep oxygen circulating to the brain until emergency responders arrive to revive the victim. Since you can't really tell if that is happening, you need to continue on with the belief that you are being successful.