I'm now halfway through Prism Tower, so I thought I could leave some tips on preparation. These are things that worked for me, but I really think most of them, if not all, are universally applicable.
1. Data types with recovery discs - you will quickly run out of moves otherwise. And at least two of each, there will be lots of doubles.
2. Forget the Ethers - don't worry about consumable items, you can't use them. At all.
3. 5GP shiny/platinum team - max EV's/IV's, shiny or platinum cards if possible, level 120-130 at least.
4. Heal Bell/Aromatherapy - If your data type gets poisoned or burned, you're screwed if you can't cure it. Additionally, being frozen or blinded overwrites these effects.
5. Moves with lots of PP - I don't mean ember and tackle, but unless you have a full team of data types, you're going to be strained for moves. I use flame wheel, thunder punch, etc.
Side Notes:
Some moves will not be restored by recovery disc, and if you have one of those moves, no PP will be restored sometimes.
I will sometimes use an enemy's ice attacks to stay frozen and recover PP for a while. A little risky, but very effective. (I call it freeze buffering)
You can save every 25 floors, but you might screw yourself over if you save in a bad position. I only save if none of my mons are fainted.
And finally, you can heal and relearn moves every 50 floors, so it's not the end of the world if you have a bad move.
Here you have it, some tips for getting up Prism Tower, from someone who's only halfway up it. I hope this is useful to someone.