There are a number of things that can be done to make the fight with Alduin (or any other dragon, for that matter) easier.
Do the Damage Yourself
Alduin, like all dragons in Skyrim, has the Dragonskin perk. This means that he takes half damage from anyone who isn't the player. Unless you've increased the difficulty, this means that allies, while useful, are almost always going to be doing less damage than you can.
Don't Take the Damage Yourself (When Possible)
In the Alduin fight, there are three NPCs attacking him along with you. They may not be able to overcome his Dragonskin like you can, but all three are marked Essential, meaning they cannot die. Let them get the first couple of hits in on Alduin, and he should keep his focus on them for a while. If he turns to attack you, feel free to run, try to find cover, and let them get his attention again. Even with Alduin focusing entirely on them, there will usually be at least one still standing at any moment, with another getting up soon.
Look After Your Friends
The NPC allies in this fight may be Essential, but that doesn't stop them from spending most of the fight in recovery mode. Directly healing them is often a waste of time, as they cannot die anyway, but buff spells like Courage, Rally, or Call to Arms can make them last longer between recoveries, and help them keep Alduin's attention longer. If you have high enough Restoration, though, Grand Healing and Guardian Circle can be quite effective. The former heals all three allies and you at the same time, while the latter creates an area that will continue to heal you over time for a minute, keeping you healthy while leaving your hands free to keep damaging Alduin.
Use Elements Wisely
Alduin has the ability to use both Fire Breath and Frost Breath, but that doesn't change the fact that he's a Fire Dragon at heart. He has 50% resistance to fire damage, but a 25% weakness to Frost, so switch to a Frost-enchanted weapon if you have one, or use Frost Damage spells. Lightning spells may also be useful, as the Dragons' versions of the various shouts they use cost magicka - drain all of Alduin's magicka, and keep it drained, and you should lock down his ability to breathe fire/frost.
Fight Smarter, Not Harder
Alduin is, overall, a giant beast made of pure pain and suffering. It's perfectly okay to turn around and run away, or hide behind cover when he attacks you. The area he's in doesn't have much, but it also doesn't take much to shield yourself from breath attacks. For more melee-oriented characters, try to stay to the side of him, attacking roughly his wings. Too close to the tail means getting caught in his tail swipe attack, but too close to his head opens you up for bites and breath attacks. As long as the NPCs are his main target, it shouldn't be too difficult to keep yourself positioned in the area he can't reach.
Use Your Outdoor Voice
Obviously for the first half of the fight, the Dragonrend shout is crucial to keeping Alduin grounded, and should be reapplied to him the moment it's available again. But, once he gets below half of his health, he will remain grounded for the rest of the fight, the same as all other dragons. Once that happens, Dragonrend is rather useless, and you can switch off to anything else you have available. Marked For Death is quite useful, as are either of the Frost Damage-dealing shouts, Ice Form or Frost Breath. If using Frost Breath, you're probably better off only using one word at a time; the reduced cooldown time makes it deal more damage overall that way. Become Ethereal is also quite useful for those times Alduin starts to breathe in, you're in the line of fire, and you don't have enough time to make it to cover.
As with basically every tough fight in Skyrim, stocking up can make the difference between life and death. Healing Potions are obviously crucial, but also poisons; Alduin doesn't have any poison resistance at all, so high-power poisons of Damage health are quite useful. Especially Lingering Poisons of various forms, since the fight with Alduin is one of the few places where such poisons have enough time to really deal some meaningful damage. Drain Magicka poisons are also quite handy, for reasons described above. Also, potions of Resist Magic, Resist Fire, and Resist Frost can make a huge difference in survival against his breath attacks.