The petstore folks are generally well-meaning, but often skimp on the details.  They generally don't tell you that it'll take 4+ weeks for this bacteria to grow.  Its the slowest damn growing bacteria that I've ever heard of.  I too was told to let it circulate for a week before adding fish.  I'm not sure how useful this is though.  It certainly won't achieve anything for the nitrogen cycle.  At least, that was the impression I naively came away from the pet store with.

You NEED fish (or another ammonia source) to get it all started.  What will happen is that the fish secrete ammonia and it will build up.  And up.  And up.  Somewhere between 1-3 weeks later, the ammonia will suddenly start dropping down and the nitrite will spike up dangerously high.  If you have fish in the tank, this can be quite a killer as they're already distressed from ammonia burn (which damages their gills etc) and suddenly the nitrite spikes up.  And then you have to wait, and wait and wait while the second group of bacteria arrive.  And hope that your fish don't get poisoned or suffocate in the meantime.  And finally, the second bacteria get a foothold and the nitite levels drop down fast.. And nitrate goes up.  Fish have a much higher tolerance to nitrate, but it is effectively algae food.  Its not uncommon for the aquarium to suddenly turn green when there is a sudden nitrate increase.  There is a nifty graph here that shows the typical progression through the cycle:   http://www.kordon.com/kpd64.htm

Anyway, this all sounds horrific for the poor fish in the tank, doesn't it?  Unfortunately, it is.  There are a few things you can do.

1. fishless cycling.  Search for this on google for a recipe, however it might be a bit late for you.  This basically has you set up a tank, and add pure ammonia and keep it at high (dangerous to fish) levels.  You have to test ammonia/nitrite levels every day and maintain the high level by adding more ammonia as needed.  This is the fastest way to have the bacteria grow. When the nitrite goes up, you cut back on the ammonia that you add because everything you add will get converted into nitrite, which will go right off the scale of your test kit.  Unfortunately, you need to keep adding some ammonia to keep that group of bacteria alive.  A few days later, the nitite will drop down quickly, and then you're done.  Stop adding ammonia, wait till it goes to zero (12-24 hours) and then add all your fish.  Note that you have to find pure ammonia for this.  I found mine at albertsons and I've seen it in safeway as well.  Its called 'clear ammonia' or 'white ammonia'.  It is nothing but ammonia and water.  Shake the bottle, it must not froth or foam - that means it has detergent in it.  And you dont want it scented either.

2. religiously test ammonia/nitite levels EVERY DAY and do whatever water changes you need to keep the levels within safe limits. (note safe, not zero.)  This is a real PITA but if you care for your fish, then you need to do it.  Some people say that partial water changes will reduce the bacteria levels and make it take even longer to get the cycle running, but if you dont want to damage your fish then you'll need to do it.

3. Get Marineland's BIO-Spira.  Except that you can't, its out of production for another month or so.  If you could get it, it is a live dose of the actual bacteria that you need for the cycle.  For this, you dump it in the tank once, along with the full fish load.  And your nitrogen cycle runs instantly.  Note that this product has to be stored refrigerated and the bacteria will die if it warms up without an instantly available supply of ammonia.  Hence, you have to add the fish with it.  Note that there are lots of manufacturers trying to take advantage of this being unavailable, but I've not heard of any others that are even close to being the same thing but are instead the lesser 'cycle accellerators' (see next item).  Actually, I think there might be one called "Fritz Turbo-start" - but this one isn't as well known in the hobby.  http://www.fritzpet.com/turbo700_main.html

4. Try out the alternative "cycle accelerators".  In my experience, these are mostly worthless and rate little more than "snake oil" as far as I'm concerned.  They vary a lot, but contain either live bacteria that sort-of help but aren't the right ones so they'll die out, or enzymes or other things to promote bacteria growth.  I'm rather suspicious of either of these strategies.  The first one bothers me a little because I imagine it could compete with the bacteria that you want.  And the second one seems to have successfully caused massive bacterial blooms whenever I tried it.  About the only thing they really achieve as far as I can see is give the pet stores more things to sell (note you have to keep adding these  things every few days for a while), and it might speed things up or reduce the damage that gets done to the fish. ("New Improved Cycle", "Stability", "Stress Zyme", etc etc are all cycle accellerators)

5. Get a jump-start supply of bacteria from a fully cycled tank.  The bacteria are everywhere in a cycled tank.. in the gravel, in the water, on the ornaments, on the glass, in the filter and/or the filter media, plants, etc.  You want things like old filter media, sludge squeezed from a filter sponge, a scoop of gravel, an ornament, etc.  As much as you can get.  The more the better.  The catch is that you don't want diseases, so fish stores are a very risky place to get it from - unless you can get it from one of their stable show tanks.  This is easier said than done.  If you're making a large purchase, its much easier to coerce them into doing this for you.  Much better would be to get some from a friend's tank.  Offer to buy them some replacements if that is what it takes.  If you can get a bio-wheel from an established tank and have a filter to use it on, take it!  Footnote:  this worked for me.  I had massive disease problems due to fish from many sources and needed to cycle some isolated tanks as quicly as possible.  I had to consider all my existing tanks contaminated.  My local fish store (not pet store) let me have some gravel, water and plant bits from one of their show tanks.  My tank cycled in 36 hours, and I used that one to jump-start the other tanks I was setting up.  Note that this combines with the fishless cycling above very well.  Dump the crud in a tank with some ammonia already in it and let the filter clear up the mess.  The bacteria will have something to feed on and produce nitrite, for the second group of bacteria.  Your ammonia should disappear, nitrite never go above zero, and nitrate start to rise.

6. Use an ammonia/nitrite detoxifier in an emergency - eg: amquel+. Personally, I'm a little dubious of the effects on the nitrogen cycle.  While the manufacturers claim that the use of these products won't starve the bacteria, they do change the form of the ammonia/nitrite so that it becomes a different molecule.  But.. I wonder how this affects the time taken to establish a new bacteria colony.  If there is already plenty of bacteria and you use amquel+ to change the molecules, sure.. the existing bacteria might consume the modified molecule to avoid starvation - but if its not the ideal food, will new bacteria still arrive to take up residence if the wrong "food" is present?  Footnote: I've just read other places that mention that this is a real problem during cycle establishment. eg: "The use of ammonia removers will only prolong the time needed to establish the biofilter."

7. Relax and be patient!  This stuff takes time and really doesn't want to be hurried.  All you can do is try and make it comfortable for your fish who really don't have much of a choice about leaving if they don't like the conditions.  This is probably the most important bit of advice I can give.  I wish I'd had this drummed into me more when I started.

I've learned a lot over the last 6 months, most of it the hard way.  I've found the dedicated local fish store (lfs) people to be generally more helpful than generic pet store people.  Many of the people working at "fish only" places are fish enthusiasts trying to support their own die-hard fish hobby :).  Actually, thats probably an important point..  If somebody at a fish/pet store is offering you advice, ask if they keep fish themselves.  On the other hand, I've also been grilled a number of times at both petco and petsmart stores by fish enthusiast employees when it looked like I was about to do something REALLY STUPID - eg: buying a tank, filter, heater, food AND FISH - all at the same time and with no water conditioner..  If they dont freak out at something like that, they're only there to sell stuff and either don't care or don't know better.  Going home with that combination is sure to result in dead fish.  That is, unless the fish are for another tank - but if they likely will not know that and they should check.