Also for Home theater, you may want to read up on some of the newer developments in that area like what the different refresh rates mean to people, LED back lighting VS Cold cathode florescent lighting (CCFL), interface options that come on the newer televisions and the different standards and revisions associated such as HDMI (HDMI 1.0, 1.1, 1.2,1.3, 1.3(rev b, c , and d), 1.4), display port, and DVI, 3D-HDTV and the differences between the different types of 3D that are available to the mass market.
Also one thing people seem to be oblivious to is the fact that HDMI is digital so it does not matter if you buy a $2 cable online or a $100 monster cable the output quality will always be the same.
There are some great articles about speaker wire if you Google that I found very useful (this is my personal favorite
http://www.roger-russell.com/wire/wire.htm)
I was in sales for about 5 years and the best advice I can give is to just try and know more than the customer.
I can guarantee people will get technical with you and having the answer is allot better than being like "uhhh let me ask my manager", and not only that you lose the customers confidence in your abilities.
I am the type of person that knows exactly what I want when I go into a store to buy something and all I can say is that most of the floor sales that I have talked with regarding big ticket purchases did not have the technical answers I needed to finalize a decision.
Good luck!!