How did you teach your Doberman to not pull on a leash? Any particular methods did you use?
Pippin, though not my "first" dog, was my first dog that I actually trained! So he got to put up with my own learning experiences, my mistakes, and my indecisive attitude.

He's been on a prong and a martingale once or twice, but I realized I had no idea what I was doing. The training class I took wouldn't allow for anything other than a flat collar or a head halter, so he was on a Halti for quite awhile. I wasn't exactly pleased with it, but it did the job and I had a trainer there helping me along. He did decently on it and it helped lay the "foundation" for actual loose leash walking, but I don't consider head halters a training tool. He now is only ever on a flat buckle collar and I still do training with it. So for now I use the start and stop and random direction change method (as well as positive reinforcement with treats and toys). When he pulls, I stop moving and he's gotta be slack before I continue. If he refuses to let the lead slacken I'll turn around and promptly move the other direction.
At what age did you start to see any improvements? How often do they need reminders?
Depends on what you mean.

Like I said, the entire time he was a pup I was learning more than he was. I would say I really saw big improvements when I enrolled him in an actual obedience class. He still needs reminders almost everytime we go out... But I don't use physical corrections. I'll give him a "eeeeey!" "eeeeeeeasy" (in a low, quiet, but not quite growly tone) when I see him starting to ignore me/move ahead. That generally works to get his attention back on me and I'll give him a command (sit, down, etc.) and give him a treat. If he hits the end of the leash I'll stop, or turn around and walk the other direction.
What type of collar and leash do you use? (prong, harness, flexi, etc)
What distractions are the hardest to overcome?
I use a four foot leather lead and a flat buckle collar. The hardest distractions are things that are alive... Humans (especially kids)... But other dogs are by far the hardest thing. Generally if I start walking away from another dog he'll start to pay attention. Humans that I don't allow him to greet can get him a bit on edge (not in an aggressive way, but he does go into a more alert mode if I continue to walk by someone who is quite aways away).
Are there any particular situations that you find more difficult to walk in than others? (Large crowds, parks, other dogs around)
Other dogs around who's owners aren't paying attention. If another dog is walking on a loose leash and ignoring him (or off doing it's own thing), he may be eager to greet but won't pull me (much, he still does need reminders). If a dog zooms right up to him and sniffs him all over it's MUCH harder for him to pay attention and to walk away calmly.