Oh, please don't use a collar and leash! That's very dangerous, you could choke her or break her neck! Use a "t" harness, like the kind they have for ferrets if she's small or for small dogs, as they dont' usually make them for cats. Practice, practice, practice, because even with the harness they can slip out. and the cat is pretty defenseless in a harness so you! must be the person that watches for other cats (we were attacked by a cat one!) and for loose dogs.
You also might want to get them microchipped if you are in an area where the SPCA/animal control uses a "reader". That way you can get them back if they end up at the pound, or even at a vets after someone tries to give them a home.
Never use a tight collar, always an escape collar, the kind that snap apart, and always put your name and phone #...don't worry about the cat name! they need your name on the tag.
And I used to keep a broom by the door. The best thing to keep cats from escaping is to have a screen door, the two give you manuevering room. Put nails or screws around the screens. A cat can get out in 3" of room,..I saw my cat get into the house with only that much opening...if she can get in, they can get out.
And the broom trick is, keep it on the side of the door where you are...so when you are coming in, push the broom in ahead of you, and vice versa...the broom keeps them occupied instead of looking for the space between your legs!