I know that quite a few people will disagree with me but I will offer my own experience as a different perspective.

My DS would cry for up to 30 minutes when he was tired and I was trying to settle him in the normal, "gentle" ways. This was pretty hard on both of us. When he was 6 weeks old a trusted friend suggested leaving him alone in a darkened room. Having exhausted all other options, I gave it a go. The first time he cried for 8 minutes, then slept. The next nap he cried for 5, then the following one only 3. To my mind, this was heaps better than 30 minutes of crying, even if he was being held for those 30 minutes. I think that my presence was too stimulating for him.

I continued using these techniques for DS2 and DD and they do self settle to sleep and have done since they were about 3 weeks old. We have lots of cuddles, I sing to them while walking into the darkened room, then I say "time for sleepies baby, off to sleep now" a few times, then say "love you" while I leave the room. If there is more than 3 minutes of crying, it generally means I have misread the cues and they are not tired. Some people would say that I just happen to have those magic children who can... or maybe I've taught them.

But you have to feel comfortable with it and be consistent. I worked for me and was best for our family.