I keep saying I'm going to write an article on this, but I keep forgetting.
Anyhow, having injured my elbows moving thousands of boxes of coins, paying attention to ergonomics has become something of a big deal for me. Studying how the body works and interfaces with my tools.... well, that happened real quick once my blown elbows let me know that they didn't much care for swinging the hammer.
Some key things to think about:
1 - Never wear a glove on your hammer hand. I don't care how tight it is, the glove is still creating a sheer plane and forcing you to grip the hammer harder as you try to maintain control of the thing. You might not consciously "feel" like you're gripping especially hard, but the thing about repetitive stress injuries is that you never feel like you're doing anything particularly awful while you're doing it. It's a tiny little thing that adds up over time. And it's important to note that not wearing a glove won't magically cure you because the glove isn't necessarily the only factor at play.
2 - Handle diameter in relation to head weight. If your hammer is under 2lb, a standard type of handle is perfectly good. Not great, but certainly okay. Over 2lb in head weight and you need to start using a sledge-hammer handle for your hammer. Cut it short, obviously.
3 - Hammer head weight and it's distribution relative to the handle. I see a lot of people clamoring for 4lb hammers because they move a lot of metal fast. Yes they do, but they also put a ton of strain on your body. Again, repetitive stress injuries are not something you notice right away. You might be feeling fine as you swing that big lump of steel, but the reality is you're putting a bunch of hurt on yourself the whole time. In the 20 years I've been forging, I've used a hammer that's less than 3lb and never felt lacking. When I use my 4lb hammers, I feel fine for awhile, but that damaged elbow lets me know real quick that it disapproves.
Unbalanced heads will only add to the problem. Consider the standard ball-peen hammer for a moment. It's design means that the main face is heavier than the ball end. This is great if you're using that main face for your work, but if you have to peen a lot - like when making bowls - you'll quickly get a sore wrist because you've put the heavier end up over the handle. This will induce twisting forces in your swing that you are always fighting to stabilize with your hand/wrist/forearm/elbow. Look at "drilling" hammers or the double-ended ballpeen hammers used in dishing and sheet metal work. They go to great lengths to make sure the heads are balanced around the eye because they are peening thousands of times in one operation.
Getting the main working face as close to the eye as you reasonably can means there's less of a lever to twist your hand/wrist/elbow with every strike on the work. Long headed hammers have their place, I'm sure, but for general work I think it's best if you keep things as compact as possible.
4 - Going back to the idea of using store-bought handles with that traditional palm swell that looks remarkably like what's on a carpenter's hammer.... stop doing it. Businesses make hammer handles by the truckload and they make them to a pattern that's general. Can't fault them for that, but you need to remember that you're an individual and everything about you is unique. Your hand will naturally gravitate to the palm swell because you're subconsciously searching for the best grip, but there's no saying that palm swell is in the right place for you, specifically. This is one of the reasons I recommend using sledge hammer handles.
You're physiology is unique, so expect to have unique needs. You won't know what you need unless you experiment. The weight of the head and the measure of control you have over it is directly related to the distance between it and your hand. Imagine a 1oz head on a 10' handle. The weight of the head is negligible on paper, but in practice it would be a real pain to forge with. Well, even a standard handle can put your hand too far away from the weight you're trying to control. And when you choke up to get better control, you're now stuck trying to grasp the matchstick neck of the handle.... which only creates its own set of problems.
Personally, I have found no better handle than the sledge hammer handle. Even though my main smithing hammer is a run-of-the-mill cross-peen I picked up at a flea market, and weighs less than three pounds now that I've ground it down a good bit, the sledge hammer handle has great thickness on both the x and y axis. I can choke up or down depending on what I'm doing, and the thickness doesn't change. The larger circumference fits my hands, but more important is that it gives me more surface area to grab onto. I'm not having to grip hard just to control the head of the hammer because it isn't thin as a matchstick and forcing me to grip hard.
Remember, every time you swing that hammer, the head is wobbling constantly. When you hit the steel, the head wants to bounce one way or another. You might not perceive it, but you can bet all that tiny little movement is being transmitted into your joints and it adds up quick.
I'm rambling so I'll end it there for the time being.
Try the sledge hammer handle and see if that doesn't make a noticeable difference. Remove the plastic coating with a wire wheel to create the best gripping surface you could imagine. Watch where you naturally grip the handle to see how it differs from where they put the palm swell on store-bought handles. See if you like the thicker cross section.
If you have to grind on your heads to bring them closer to the handle, do it. You'll be happy you did.