I suggest that the center chest "d" ring on a harness be used only in directed training sessions. Although I can only speak for the Walk-Nice harness that I make, the techniques should work using any harness front "d" ring or center back "d" ring. I do not recommend using the center front "d" ring for a causual walk - the purpose for the front "d" ring is to help train the dog to walk nice and not be a leash puller.
Instead of going for your regular walk, you go for what I call a crazy walk.
For these training sessions you might want to find a private area so your neighbors won't think you've gone whacko. Start off by walking forward,but before your furbaby gets to the end of the leash turn and walk in the other directon. You never want him/her to actually get to the end of the leash and pull before you turn and walk in a different direction. Turn left, turn right, do a 360, zig zag, etc. Use a shortened leash and hold it out by your side to help your guy circle around. Timing is of the utmost importance. You want to time your turns and zig zaggies so that you change direction just as he/she gets to the end of the leash and issue a verbal command as you change direction - say something like "no-pull" or "walk-nice". It won't be long before all you have to do is say the command to remind your furbaby to not pull on the leash.
Work in short sessions several times a day if possible
always using excessive praise whenever he/she is walking nicely and not pulling. If you are consistent, it won't take long for them to get the idea that walking nicely and not pulling on the leash is more pleasant.