...and if you aren't quick enough and you don't use a big iron you will end up with cold joints. A couple more tips:
don't use too much solder and have fast hands.

It's an aquired skill, but it's pretty easy to get the hang of it. I'm fairly certain this is how everybody does stick packs. Even the competition guys.

I will say that I check all my joints by trying to break them with my hands right after soldering them. Every once in awhile I get one to break and when this happens the leverage I get on the cells makes it's pretty easy to break the cells apart. If I have to struggle much to break them I consider them well soldered. I know it's not scientific, but it's all I got and it's served me well.

Cheers

Wiz