I agree with Cloud Strife, this was my favorite fight in the entire game.
Use Metal Chocobo or Olympia.
Equip "Guard," "Glide" and "Ars Arcanum" at the very least.
Keep "Cure" and "Aero" on your shortcut menu.
Stock mostly Ethers in Sora's menu, and maybe an Elixir or two.
Remember the #1 rule about fighting Riku...good or evil, he talks too much during battles. What he says is always a big tipoff as to what he'll do next. For example:
"Ha ha ha!", "Useless" or "Heal This!" - attacking
"Come!" or "SHOW ME YOUR POWER!" - blocking
"Behold the power of darkness!" - powering up/Dark Aura move
"Is that it?" - trash talk (a good time to heal/restock MP)
Cast Aero to start with, Guard his attacks (or strike at the same time) and combo him, but be careful with the timing. Ars Arcanum with Olympia really does a number on his HP. I tend to Glide or Dodge Roll away from his Sonic Blade since I don't like messing with it. He occassionally does his "block three times & attack" pattern from Destiny Islands, but know what to do there. This would be a good time to heal, since he's just standing there. But make sure you're far away or he'll get impatient and come after you eventually.
When he starts glowing (powering up), Strike Raid him to cancel it. Or use this time to heal/restock. If you're lucky, you can mangle his lifebar down pretty far before he starts with the heavy stuff.
Eventually he will power up and all his attacks will do Dark damage. Plus he uses "Strike Raid" (at least that's what it looks like; throws the blade, zips behind you to catch it.) Guard, smack, repeat.
When he starts with the move now known as "Dark Aura," (teleporting Sonic Blades, ending with a big exploding finale) Glide around the edge of the fighting area. You may get hit with a glancing blow here or there, but it should not be anything serious. As long as you stay out of the center, thus avoiding the "big finale." If you must heal, heal immediately after Dark Aura's finished.