Definitely use flux, and if the solder doesn't flow, add more flux. It's not necessary to sand the inside of the terminal so it's bright, you're chemically cleaning it with the flux. I'm suprised that no one mentioned solder pellets for the big closed lugs? Or tin/solder pots for tinning wires? Tinned wires aren't always the answer though. If you twist em round and tin them for insertion into your amp outputs, you ultimately give up a lot of surface/transfer area versus a flattened cable, tinned or not. If you leave your twisted up cables bare, they flatten and spread out easier.