These little guys can be quite freaked out when being moved around. They may thrash and jump and end up with injuries. It is best to move them as fast as possible. Try not to handle them much. When you get your frog into it's new habitat leave it alone for a day or so to get used to its surroundings.
When you get your new frog it's a good idea to quarantine it for about a week to make sure it doesn't have any illnesses or diseases before putting it in your tank with other frogs.
Overcrowding is a bad thing. It makes the tank too small as well as your frogs unhappy. They could also become injured from running into one another as well as the tank sides. You want to make sure that the frogs you have in your tank cannot become another frog's meal.
If you find that one of your frogs is sick, immediately take it out of the tank so that your other frogs won't get sick.
Xenopus Husbandry - This website is VERY useful but very technical. It covers a wide range of ailments that your claw frog could suffer from and includes the remedies to help.
African Clawed Frog Facts - This website has great pictures and alot of useful info in layman's terms. Great website for new owners.