Sammy loves her harness and leash. We don't let her out any other way, and I still keep her close to me when she is out. She starts purring immediately once I start putting the harness on her! She's only allowed out on the porch though.
The keys are to:
1. Get a very well fitting sturdy harness and leash. Just like a collar, you should be able to fit 2 fingers comfortably but snugly underneath the fabric/leather of the harness (especially around the neck and chest), no less, and not much more.
2. Make sure your cat has their tags and/or is microchipped, just in case.
3. Make sure your cat will tolerate the harness and leash. Many will act as if you've just attached a 16 ton weight to them and they won't enjoy being outside with it (the low-walkers).
4. Make sure your cat will not slip out of the harness at all (follow-up to step 1). They're slippery little creatures.
5. Supervise supervise supervise. If you don't want your cat to run away, chase the squirrels, get chased by the neighborhood dog, or get hit by a car, you have to be out with them at all times.
Also remember, sometimes the grass just looks greener. Some cats just enjoy seeing the great outdoors but not being in it. Plus, keeping cats indoors keeps them safe from injuries and illness. Who knows what pathogens are lurking in that pile of leaves Fluffy keeps sniffing.
If you're really set on walking your cat and they are a low-walker, try leaving just the harness (not attached to a leash) on the cat for an hour or so around the house. Give him/her treats right after putting it on and right after taking it off. See if things get better after 10 days of this; if not, Kitty's probably not going to enjoy walks.