Its the yeast settling in your bottles that drags down the hop aroma compounds. Yeast are sticky buggers and the compounds adhere to them. So your hop aroma hasn't really disappeared. It there, but sitting at the bottom of your bottle. This might also go some way to answering the question as to why the above mentioned James Squire beer can sustain its hop aroma for long periods - I assume this commercial beer is filtered before aroma hops are added. With no (or very few) yeasties to grap hold of the hop aroma compounds they are left for you to enjoy when you crack one.
The sort of filtration equipment used by the big boys is beyond most homebewers. Anyway, you need yeast in your beer if you are letting it naturally carbonate in the bottle. So some other ways we homebrewers can maximise the retention or longevity of hop aroma could include allowing more of the yeast to settle in the fermentor before dry hopping. The more yeast that are still in suspension in the beer the more aroma compounds that will be dragged down with them when they settle. So leave dry hopping to later (although some might argue that you need a little bit of active fermentation to keep the hops mobile in order to extract the most out of them).
You could also look at using a more flocculent yeast (one that naturally settles out quickly). The popular US05 is not a very flocculent yeast and stays in suspension within the beer for ages. You could go to something else like Nottingham (there are many other hi floc yeasts), but it may change the flavour profile of you beer (not in a bad way, just a bit different depending on the yeast).
Another thing to look at is your bottling process. The more air (oxygen) you trap in the ullage space at the top of the sealed bottle the more yeast will be produced. The presence of oxygen encourages the yeast to reproduce. They will scavenge the oxygen in the process but will increase their numbers, and the more yeast there are the more aroma compounds will be will be dragged down to the bottom of the bottle when the yeast settles. So try filling your bottles and leave the cap loose on top for a time to allow the CO2 in the partially carbonated beer to flush the air out replacing it with CO2. Then cap. If the beer coming out of your fermentor is pretty flat you could try giving the loosely capped bottles a swirl to cause it to outgas what little CO2 is has.
As mentioned by Tugger above, perhaps the easiest way to extend the longevity of hop aroma is to add more when dryhopping. Expect your sample to smell overly hoppy after a few weeks, but they might be just fine at twelve weeks.