OK, this is the second kid that I have potty trained in one day, so I know it's not just a fluke. Therefore, I feel obliged to share my secret knowledge with the entire toddler-raising world.
Step 1. Understand your child's physical and emotional abilities based on her age and maturity. Make sure your child can do the following: click here for previous post's list. Spend time familiarizing kiddo with the concept of potty, let her use potty when she's interested, watch others using potty, read books about potty, etc.
Step 2. Decide you want to make the plunge because there's no turning back, not even for a you've-already-been-to-the-potty-fifteen-times second.
Step 3. Know your child's currency. Every child has a currency... a thing that they love above all other things. This currency can be used to shape behavior. Instead of yelling, screaming, spanking, etc., use your child's currency to reward good behavior and to reform negative behavior. Drew's currency at potty time was the Zamboni. It has of course changed over time, but in order to have effective consequences for actions, it is pertinent that you follow their trend and mold your behavior accordingly. Maddi's currency is babies. So, we went to Toys R Us together and bought the doll of her choice. Before purchasing, we talked about the baby. This baby was only for big girls who used the potty, so if she wanted to play with this particular baby, she had to use the potty from now on. Anticipation and excitement for our new goal was palpable in the check out line. This was going to be so fun, and she was so ready for the challenge! Baby doll came home and sat on the counter within toddler eye-sight, waiting to celebrate her big achievement with her.
Step 4. Overflow the poor kid's kidneys. Push juice, milk, and popsicles all day long, and ask them a million times a day if they need to go. This gives them the maximum opportunity for success.
Step 5. Get every one involved. When kiddo uses the potty, throw a PaRtY. There must be clapping, singing, marching, phone calls to distant relatives. Be ready to make a fool of yourself :)
Step 6. Once you've started, don't turn back. I personally go straight to the panties, no pullups or padded panties. It's all or nothing. You must also take the little person to the potty every single time they ask. Yes, this is time consuming and a little annoying (we went to the bathroom 4 times at the library and seven times at work the first day) but this isn't just a physical necessity, it's building the foundation of trust in your eternal relationship with your child.
Step 7. After your successful day (both of my kids had no accidents the very first day in anticipation of their new toy), pay up! Give them their reward and celebrate with them.
That's it. One day. Let me know if it works for you too!