I have now potty trained 2 boys with 2 totally different personalities. Here's my brief manual.
I agree with the parents who say there is a window of opportunity for a child. He must be willing to cooperate, can verbally express their needs, and can physically assist in the process with pulling up and down clothing (most importantly...pulling down). If you wait until the child is too old, the window closes and they understand that NOT going in the potty is POWER.
My oldest was delayed in talking. This caused more tantrums and less cooperation at times. The grandmother tried training. FAIL. We eventually waited until he plainly outgrow the pull ups. With a child who is 95th percentile in height and weight, this means about 3.5 years old. We simply explained that 1) We had no more and 2) He was too big and they didn't make larger sizes. He said "ok" and was dry ever since. There was no additional effort on our part as parents.
With the second child, he was speaking well a lot earlier. He was also staying dry for longer periods of time. And more importantly, he was going to the bathroom at regular intervals and being successful in all attempts.
Here's what I did to prep:
1) Get underwear with the favorite cartoons (Cars 2 and Thomas the Train). I bought 2 packs of 5. This gives you a lot of underwear to soil if necessary without having to do laundry every 30 minutes.
2) Buy extra light colored pants. If you are potty training in the cooler months, it's difficult to have a child run around the house naked or only in underwear. Gray was my color of choice. I could see the smallest accidents from across the room.
3) Have treats handy. Depending on the child's temperament, you may or may not need to use this tactic.
4) Any sliver of urine on the pants. Tell the child "oh you pee-peed. we have to go to the potty." Pick the child up and race to the bathroom.
5) Be aware of the child at all times.
6) Consult with other mommy friends with very young children who have potty trained a child successfully.
I expected to train for 3 to 4 days. I picked a long holiday weekend so I would be available to be eagle eyes. On the fourth day, we went on a small outing to see how he would do in a public place.