Here is some interesting info we found at Parenting.com
Potty Training Problems to Watch For
Constipation. Sometimes children will hold in bowel movements rather than go in the potty. When they do, the stool gets drier and harder, and more difficult to squeeze out. This makes pooping painful, which can make the child even more hesitant to do it. To avoid this, make sure your child's diet includes plenty of fiber. If he's really holding stool in for a long time, it may be a sign that you should let him poop in a diaper for now.Urinary-tract infections (UTIs). These can occur when your child's not urinating regularly enough, or because of poor hygiene (usually in girls -- make sure you stress wiping from front to back from the start!). Prevent UTIs by making sure that your child gets plenty of fluids and doing the wiping yourself (or having a trusted grownup do it) for the first few months.
Potty Training Problems to Watch For ,By Dr. Claire McCarthy, Parenting
Potty Training: Girls vs. Boys
Most moms potty train in hopes of (finally!) being able to walk past the diaper aisle. But the difference between teaching boys and girls can seem like night and day. Both genders begin by sitting, but boys eventually learn how to stand and aim. Girls learn more quickly but have to figure out how to position themselves and wipe correctly.
To train boys and girls in their own special way:
Try using a potty chair -- her feet will touch the floor, which relaxes her pelvic muscles. If she's using an adult toilet, give her a step stool.
Limit spray by having her sit all the way back so her bottom and vagina are over the potty opening. Encourage her to sit with her knees apart, which will also hep relax her pelvic muscles.
Teach her to wipe (or pat) from front to back by letting her watch you. Keep her entertained and sitting with books, stickers, or music next to the potty.
Let him use a potty chair to pee if he's not ready to stand and aim. For pooping, use a potty chair or toilet-seat insert (plus a step stool).
Have him push his penis straight down before he sits on the potty chair to avoid scraping it on the splash guard. If he's standing, be sure to position him, feet slightly apart, directly in front of the potty.
Have him watch his dad, or show him how to aim his pee into the bowl. To improve his aim, drop a few Cheerios or goldfish crackers in the toilet, then offer him a fun sticker for hitting a few. If he's making poop, give him a book or play some music to encourage him to sit.
Potty Training: Girls vs. Boys, By Jessica Kowal, Parenting