These are the cases that we recommend/ use:

Protection is the key and there are hundreds of cases to choose from. Try Amazon as your starting point. Here fairly representative ones for £4.35 (left) and for £32 (right):
These cases do protect well but they can make your iPad bulky.

There' s nothing to beat the Apple smart cover. Cheaper lookalike alternatives on Amazon but I hear that the quality isn't as good. You will also need to protect the back from scratches with a Gelaskins.
An alternative for those who don't like to wrap their device in padding is to combine a Gelaskins with a cheap carry case that you can get for under a fiver:
You could also invest in a hard case, either for the back or both back and front. The one shown is £1.50:

If you want a good investment why not buy a case that doubles as a keyboard? The one on the left is £25 and the one on the right £70. it makes extended writing entirely possible.