Before dry hopping in the secondary, rack off the trub in the primary to remove as much yeast mass as possible.
Try to keep your beer as cool as possible while dry hopping. In the low to mid 60's °F (16-20°C) is best.
Since we are now past the aerobic phase of fermentation, make sure
to minimize the amount of oxidation to your beer. I like to put a layer
of CO2 over the surface of my beers when doing any type of manipulation
after the primary. And dry hopping definitely falls under the
"manipulation" definition. We are not trying to keep oxygen from the
beer per se, as much as we are trying to keep oxygen away from the
hops. We definitely don't want to oxidize the hop compounds here.
Better safe than sorry.
Just like you would before you rack from secondary to a serving keg,
crash the vessel you are dry hopping in down to between 32-36° F
(0°-2°C) to drop the hop mass and get as clear of a beer as possible.
You will get better extraction if you can stir the beer (without
adding oxygen) to keep the hops in contact with more of the beer in your
To reduce the hop mass, you may want to use a hops bag, especially if you are dry-hopping with pellet hops.
References: Information for this article was adapted from the Brew Your Own Magazine article by Donald Million entitled Dry Hopping: Techniques online date of August 26, 2003, the book
Homebrewing: Volume 1 Beginner Basics to Creating Your Own Award-Winning Recipes, by Al Korzonas, the paper in Chem. Educator 2000 entitled Beer: An Ancient Yet Modern Biotechnology written by Charles Bamforth, Department of Food Science and Technology , UC Davis and for the dry hopping tips, Brewing Better Beer: master lessons for advanced homebrewers written by Gordon Strong.