As these other guys said, most of these products are garbage. How does the beneficial bacteria live in these products if it needs insane amounts of oxygen? If your filter turns off for a few hours, you can "break" your cycle and have to start building BB from scratch, but these products can sit on a shelf in a sealed container for a year at a time? Hmm. . .
Seed filter media. That's a great way. I keep a sponge filter running in at least one of my tanks at all times for exactly this reason- any time i set up a new tank, I just pop that whole sponge filter straight in the tank. It's a completely existing colony of BB already in a filter.
The only store bought products I ever use (asides from Bio Spira, as someone mentioned- i hear that stuff is great, and would try it if I needed an instant cycle) to help keep new fish alive is zeolite. While I don't recommend regular use of zeolite, if you use it as the last media in your filter (mechanical, biological, THEN zeolite, compared to mech, chem, then bio), it lets your bio media process as much ammonia as it can, then it absorbs whatever ammonia is left in the water after. It prolongs the complete cycle, but it will keep fish alive if you're in a pinch (like when power goes out and you kill your BB, of it a pump breaks & goes dry or whatever) and you're forced to start over.
