I copied this years ago from the old forums:
The ideal playthrough @Lips posted is a pretty solid way to do it; I played through using that method once and it was a nice experience. Since then, I’ve modified it a little to satisfy the completionist in me:
1. Follow the ideal playthrough in PT1 main story.
2. Complete Prove Yourself in PT1 for the skill point.
3. Play Dr. Ned in PT1 up to rescuing the Claptrap for the backpack SDU (avoid all side missions except the Claptrap Rescue).
4. Complete Robolution in PT1 for the backpack SDU and skill point (again, avoid all side missions except the Claptrap Rescue).
5. If you’re a glutton for punishment or really want 72 slots at the end, play Knoxx in PT1 up to the Claptrap Rescue (again, avoiding all other side missions) and farm it until you get your Backpack SDU.
6. Fire up PT2. Jump right into Ned and complete up to the Claptrap Rescue.
7. Do the same for Robolution (stop at the Claptrap, instead of completing the the whole thing).
*8.The reason for doing these two is that once you find Steele in the main game, these Claptraps won’t reward you with an SDU. Ever.
9. This will put you at a point where you can pretty much skip every side mission in the main game. So run through the PT2 main game, avoiding side missions almost entirely.
10. Complete Knoxx PT2.
11. Complete Prove Yourself PT2 for the skill point.
12. Finish Robolution for the skill point.
13. At this point, do whatever the hell you want.
That’s how I did my last few characters. You’re a bit overleveled right at the beginning of PT2, but skipping the side missions lets the game catch up with you pretty quickly.
I should say that I am in PT 1 and running the GotY Edition. If I understand it correctly, I have to stop the main game before running into Steele on the Eridian Promontory, and do the DLCs, up to the Claptrap rescues—otherwise, I cannot get the SDUs from those DLC missions? If this is correct, it seems like a bug that wasn’t addressed. Also, if correct, it seems like a great way to prevent players from enjoying the natural flow of the game!
Is this the case, or is the author, above, mistaken? (Or, am I simply not understanding what this guy wrote?)
I’m actually on the Promontory right now, just before running into Steele.