Before I solder my cells, I normally add an extra shrinkwrap sleeve to each cell. Keeps the matchers labels looking clean and keeps the cells from shorting out easily on graphite chassis.
Self-Tinning flux will help alot. I just use a 60w Hakko and it does great to sloder cells.
I tin the cells first then put them in the jig. I take the battery bar and put just a little flux on the top and bottom and place onto of the cells to be soldered. Apply the iron and the battery bar will suck to the top of the cells because the solder is flowing into the pores. If you pretinn the bars with solder sometimes you get a hard melt and the solder does not melt and flow properly between the bar and battery.