Sabin is arguably THE best fighter in the early part of the game. It's funny, because players rarely see him as more than that. Many have even said that he's overrated because his last blitz, Bum Rush, isn't as good as some physical attacks you learn for other characters later on in the game. It's true. I'm not going to lie and say that Bum Rush is the best attack ever, but if you plan out how you're going to use Sabin early and stick with it, you're going to have a very solid character in your party.
One of the many things that make Sabin awesome is that he can be used in three different roles: fighter, black mage, and white mage. Almost everyone uses Sabin as a fighter, but I bet people rarely think of using him as the other two. Let's look at each.
Sabin is most likely used by most players as a fighter. He's a martial artist, he's bulky, and he has super-human strength. Perform Suplex on the Phantom Train and you'll see what I mean. It makes sense that he'd be a fighter. During the early (pre-esper) parts of the game, it's easy to use him this way. Pummel is a solid physical attack, and one of the strongest you'll pick up early on in the game, and Suplex is pretty decent when you're facing just one foe because it deals double damage. You can get pretty far on those two blitzes alone. Sabin doesn't have great defense, so you might as well make his hits count, and hit hard with blitzes. I would use nothing but blitzes with Sabin when using him as a fighter, as they are going to hit the hardest.
Because Sabin's defense is lacking, you'll definitely want to take steps to help him out with that. Put Sabin in the back row, give him a shield (I like giving him the Genji shield.), and it wouldn't hurt to give him some better gear as well. The Genji Helm is great.
You'll also want to raise his strength. As soon as you're able to get espers, find Ifrit, Bismark, or Raiden, and equip it to Sabin! Those espers give strength boosts with every level Sabin earns. If you're planning on using Sabin as the monk he is, this is your best bet for making him good. Level! Level! Level! Equipping earrings and a black belt to Sabin will also boost his strength and attack. There is other gear that will boost Sabin's strength, but at the expense of his defense which already sucks, so pay attention to the stats of all equipment before using it on Sabin. And while we're on equipment, don't buy weapons for Sabin. Blitzes don't need them, and you should be using nothing but blitzes. It's my experience also that his fists are stronger than any weapon you can buy for him, at least until later on in the game, and I don't think it matters then.
In my opinion, Sabin is best used as a black mage. Most of his blitzes have elemental properties that make enemies that are weak to certain magic weak against them. What's more is Sabin's blitzes don't consume MP! This is huge, as Sabin doesn't get much MP. Fire Dance, Air Blade, and Bum Rush are fantastic blitzes that will get you through almost all of the later part of the game. AuraBolt is also pretty solid in the early part of the game; a lot of enemies are weak against it.
When using Sabin as a mage, I would definitely get Bum Rush as soon as I could. That means you're going to have to make a trip to Duncan's house. It's worth it, believe me. It definitely beats waiting til level 70 for it. Once you have Bum Rush, you're going to be set with Sabin. There's really no need to use any other blitz once you have it except for AuraBolt which I'd continue to use when you're fighting more than one enemy. Bosses, though? Attack the crap out of them with Bum Rush.
In addition to his blitzes, Sabin can learn some pretty mean magic like Thunder through espers. I would try to focus on the magic Sabin doesn't have through his blitzes so that you have a well-rounded mage. You're also going to want to increase his MP by equipping espers that raise that stat and leveling him, because like his defense, Sabin's magic stats aren't great. It's going to require a ton of leveling on Sabin's part, but you're going to have a lot of time to level Sabin with espers, as he's readily available then.
Remember above where I said don't waste money on weapons for Sabin? You're going to want to ignore that if you're using him as a mage. Get him Tiger Fang as soon as you can. It's arguably his best weapon (Some say Godhand in the GBA version is.), and I prefer it because it raises his magic stats. Also give him a Circlet to boost his damage.
Equip him with some armor to boost his defense (The Red Jacket and Genji Helm are great.), and put him in the back row.
I'm putting this in here because it is possible, but I do not recommend making Sabin one of your main white mages. I think his white magic blitzes are some of his worst, and other characters benefit more by learning spells like Cure. If you do insist on making him a white mage, though, I guess just for kicks, apply all the defense tips from the sections above. Put him in the back row. Give him a shield. Replace his gear with something with better defense, but pay attention to how they affect his stats.
When it comes to leveling Sabin as a white mage, Bahamut would be the best esper to equip to Sabin. It raises his max HP by fifty percent. A high HP is imperative to using Sabin as a white mage because of how his blitz Mantra works. Mantra is the only blitz I'd use with Sabin, too. Spiraler takes Sabin out completely, so if you decide to use it, make sure you're not going to need Sabin for the rest of the battle.
Ideally, I would use Sabin's white magic blitzes on a need-to basis. Characters like Celes and Locke are better suited as main party healers in the game, and I think you would just be wasting Sabin's strengths by using him as a white mage. If you use him mostly as a black mage, and use his white mage blitzes when you have to divide the team up and need an extra healer, you're going to be better suited for anything that comes along. Sabin's strength is in attacking, so use him for that. Raise his magic stats with espers, and if you have the time to just walk around and level a bit, equip an HP-raising esper like Bahamut so he'll be a better healer when you need one.