If the wiki gameplay page is correct (or perhaps I should say, if Arcorann is correct, given the page history): Auto-collecting 20-59 items at once gives you a bomb part, while auto-collecting at least 60 items (a 2.0 multiplier) gives you a life part. In addition, bosses drop a bomb part after every phase you don't die or bomb on. Every 5th bomb part (from either method) is replaced by a life part.
Oh, and SakuyaA's bomb is like SA MarisaC's bomb, giving you 3 bomb parts back if you don't break the border. Those 3 bomb parts don't count towards the life part.
I wish there was stage practice too.