After toilet-training three children and discussing at length (for hours and hours and hours) with friends doing the same, I feel it is one of the most challenging aspects of raising young toddlers. Though each child is very different, here are a few random, essential tips that helped me over the years:
1. Be positive. This is The Most Important Tip. Totally Positive. Even a sad sigh of disappointment will set you back. Never let your child know that you are disappointed, sad, mad, frustrated, tired-of-cleaning-up-accidents-annoyed, etc., - even when you know the child intentionally had an accident, (this is so, so hard, but so, so, so essential). Continually encourage your child; make sure he/she knows that you believe in their ability to accomplish this huge feat. Otherwise, they will resist it - wouldn't you?
2. Don't give up. (And you will want to, believe me, over and over again. You will see that box of diapers in the closet and be tempted to go back. Don't do it. If you do, it makes it even harder to start up again later - the child will remember the previous experience and will not be nearly as excited about it.) Take a deep breath before you start, and stick with it.
3. Be consistent. You are teaching your child to develop a habit. That is all. You will be so focused on them developing this habit, that you will become very self-disciplined yourself. If you are lax, even for one minute, they will figure out that they can be lax about it too (switching between diapers and underwear will just invite confusion). It will take precedence over Life for a while, but you and your child will both be so much happier when the habit is formed.
4. Don't allow it to become a power struggle between you and your child. This usually stems from the child recognizing parents' reactions and thinking: "mommy is happy when I use the potty, and sad/mad when I have an accident, hmmmmm..." The child can then use this knowledge when they are mad at mommy/daddy, etc. (this is with older children). Just revert back to #1 if this happens. Also, don't celebrate the successes too much; reinforce the fact that this new habit is important for the child because they are now "big," and that it will make them happier, not you.
5. Buy a simple, fun, easy-to-clean potty. We bought "froggy" for $12 at Target. Love it. (Sorry, I couldn't find a link for it.) It's low to the ground, totally not intimidating, prevents spills, and super easy to clean (the most important aspect for mommy).
6. Buy lots of fun underwear for the potty-trainee. And lots of whatever-you-use-to-clean-up (chlorox wipes, cleaner to spray and wipe, etc.) to lessen your stress when accidents do happen. for. they. will. happen.
** If the child has older siblings, the entire ordeal will be so much easier and faster (usually).