Even though I've only been brewing for 2 & 1/2 years, dry-hopping seems to continue to grow in popularity among both homebrewers and commercial brewers. It's a technique that is well-recognized as adding an extra smack of hop aroma that you can't always get from late boil or whirlpool additions. It's very easy to do, and can really make your American IPAs and Pale Ales taste that much fresher and brighter. You'll see a lot of other beer styles lately that employ dry-hopping, including American Amber Ales, Pilsners, English Bitters, and various Belgian Ales.
The basic technique for dry-hopping is fairly easy... you dump the hops into the fermented beer. You don't have to sanitize them before doing so; infection is just something you don't have to worry about when dry-hopping, as a lot of sources note. When I'm weighing them out onto some aluminum foil, I'll spray the foil with a bit of sanitizer and wipe it down, but even that probably isn't necessary. Just try to keep oxygen contact with the beer as minimal as possible. Despite the general simplicity of dry-hopping, however, there are some things to clear up:
Whole hops vs. pellets
I've heard some people argue that hop pellets give a more grassy aroma to the beer, but I personally have never found this, and there's a lot of people out there, amateur and professional, that use pellets for dry-hopping. If you use pellets, you can just throw them into the fermenter... they'll turn into a green sludge soon enough, and eventually start to break-up and sink to the bottom. Giving the fermenter a twist back and forth can help speed this up, and will bring the hops more in contact with the beer. Whole hops work fine too, but they should be placed in a sanitized mesh-bag so that there isn't a lot of hop particles circulating through the beer. Makes them easier to remove, as well. Or, you could cold-crash at the end of the dry-hop to help clear the beer up.
Amount of hops to use
Obviously a personal preference. The more hops you dry-hop with, the bigger the aroma will be, and likely the longer it will last. Some brewers go really big and make dry-hopping the largest of their hop additions. For example, the Can You Brew It guys did a clone of the fantasic Lagunitas A Little Sumpin' Sumpin' over a year ago. That recipe had 59 g (2.1 oz) of different hop varieties throughout the boil. The dry-hop, however, is more than double that, at 129 g (4.6 oz)! I brewed that clone a while ago, and it did indeed have a huge hop aroma. However, even using 14-56 g (0.5-2 oz) of a single hop variety can add a really nice, bright hop aroma to the beer, especially when used in addition to a late/whirlpool addition in the boil.
When? Opinions vary... when primary and secondary fermentation is absolutely complete, and you're close to bottling/kegging? When fermentation is slowing down, and FG is 1 or 2 points away? You can try both options and see which suits you, but it's generally agreed that fermentation should be at least MOSTLY complete. If you added dry-hops when fermentation was very active, a lot of the aroma compounds would simply be blown off and released with the CO2 produced by the yeast. If you keg, adding hops after you've transferred to the keg is supposed to be a great option. A lot of brewers will add one hop addition to the keg for several days, and then add more hops when they're about ready to start drinking the beer, to allow for extra hop-freshness.
How long? Most agree that about 7-10 days is a good period to dry-hop - long enough to get a lot of extra aroma into your beer, but not too long where the hops may begin to take on an overly-grassy note. This is why I personally like to wait until the beer has been in primary for at least 2 weeks, so that the yeast have a chance to clean up their byproducts before bottling.
Primary vs. secondary
This is a tricky topic, at least in terms of whether dry-hopping at fermentation temperatures vs. cold-crash temps actually makes a difference, aroma-wise. Dry-hopping warm (60-70 F) takes less time, but dry-hopping after cold-crashing (to ~32 F) will help hop particles settle out quicker and more efficiently. Some have suggested that cold-crashing the beer before dry-hopping also
leads to more hop aroma, because the yeast will floc out quicker, and
therefore won't be in suspension to rob the beer of those precious hop
oils. I've seen recipes call for either, and the Epic Pale Ale clone I brewed two weeks ago calls for both: one addition for 5 days at fermentation temps, cold-crash the beer, and then add more hops for 5 days at the cold temperature. This approach does make sense, as a double-hit of dry-hopping would add more aroma.
One question that may need to be asked is if dry-hopping, like other brewing techniques, is sometimes OVER-used. Maybe. There doesn't seem to be a ceiling on hop taste or aroma like there is with bitterness, so using too much hops could be a possibility (although with today's increasing number of hopheads, I doubt it). I think it's more likely to be a problem if a brewer uses too many hop VARIETIES. While it's nice to mix different hops to see if/how they complement one another, when you get into 4+ hop varieties, the flavors and aromas can get a bit muddled. If you're just getting into dry-hopping, try limiting yourself to 1-2 different types of hops to start, and then work from there. It may be a good idea to split the beer into two batches after fermentation is complete, and dry-hop each batch differently, so you get a better feel for what separate hop varieties bring to the table.