Sorry to hear that you're having a hard time of it. There's a couple of guys on this forum that make their own. Here's what I know.
1. Use a hot soldering iron (mine goes up to 65W)...you want to solder things as fast as possible without heating up the cells. The solder should melt instantly, remove heat and let things cool down before moving anything.
2. I've only used 600 grit wet/dry paper to rough up the terminals before soldering...it's just what I have on hand.
3. Get everything ready and laid out before you start soldering. A little jig or vice is handy for holding everything in place.
4. On the packs that I've built, I prep the terminals (I think that it's called "wetting") with a little solder that I let cool before going for it.
5. I also prep any ends of wire with solder.
6. Then I just go for it and make sure I don't have one the cells backwards
Other than that...I can't think of anything else. I don't do a lot of battery packs, just things to get me by in a pinch.