Taking outside with leash and harness?

Molly and Polly are indoor cats, but Molly's taken a real interest in the outdoors. I have to be careful when I open doors. I usually pick her up before doing so, because she is so incredibly fast and darts out, then I have to get her. Each time she's gotten out, she's more obsessed with trying to get out another time. She just realllly wants to go outside.
I was thinking maybe I could start taking her out on a leash and harness so she could safely enjoy the outdoors. I'm just afraid when she gets to go outside she'll want out even more and I'll have more trouble keeping her in when she's not supposed to be out.
Do any of you have any thoughts/advice?
Thanks in advance.

My 3 all go out on harness, they quickly learnt that harness=outside. None of them try to escape, I can have the door wide open and they'll just sit near the door way.
Any time I open the door I say "back" and they've learnt it means to take a step back allowing me to enter.

I've tried to put a harness on my baby but she got completely freaked out & ran away

I have harness trained every cat i have had. The older a cat is, the longer it will take. I did it on a 4 yr old cat in about a month. One thing i found is that you have to get them eating in the harness. They willl freak out the first bit, but gradually increase the time they wear it, and things will happen ok. Spice wears her harness all the time. She knows that when I grab the leash and say wanna go outside, she will be permitted to if on a leash, I say back and she doesnt go. Just be simple and firm with commands and they will learn,. Also remember to use the same commands, and reward them too for good outings. Good luck, Spice loves it, and loves rolling in the grass and on the pavement.

My mom takes her cat, Curly, out in the yard on a harness and leash all the time. He knows the certain times when he can go out. He loves it. And he's so good with her putting the harness on.

Both of mine are harness trained and they love being teathered and walking around the yard. I first got them used to a regular collar, and then a harness. They walked around at first as though they had a 100 lb. pack on their backs, but they adjusted. I started by just a minute or so with the regular collar and worked up from there.

Be sure to stay with them. Mine "wander" on a leash too. (Walking a cat is different than walking a dog!) They seem to like the teathers better b/c we're there to protect them, but they have more freedom and we aren't right on top of them. They get to stalk birds that come to the feeder when we're in the yard but not right with the cats.