Basic psychology is about people's needs and their need to fulfil them. Most of us have a distinct priority at any given time, one problem that must be solved before all others, a "one thing at a time" mentality.
You will be unlikely to interest a homeless man in a new car, until he's fulfilled his basic need for shelter first - unless he plans to live in vehicle, I suppose.
Understanding this principle and seeing how it applies to selling, will enhance your sales performance zillion-fold. Sell one thing at a time and concentrate on that. Sell other things as "back-end" once immediate need is fulfilled. Don't confuse, don't stray from point, do stick on blinkers and keep to target and matter in hand.
Don't overlook obvious.
Make sure you put words "Click Here" on a banner. People respond to simple commands. Serve up testimonials next to your products or your newsletter sign-up form. People want to belong to groups: they'll want to belong to your "club" if it is seen to be a good one and endorsed by others.
Use colours that convey right image and incite right actions. Want to be seen as an authority? Use black and yellow. Conservative and business-like? Dark blue, maybe a bit of grey. Business-like and money-orientated? Blue and green - which so many large corporate sites have adopted.
Positioning of elements on a web page also makes a big difference. Apparently, whatever is at top-right of screen is what's most likely to get clicked. The average eye is drawn to that position - nothing to do with Windows' exit button being up there, I'm sure! This and 90% of population being right-handed, to me, makes a strong case for right-hand navigation, with your best offer in pole position.
Words: I could write tomes on what you can do with them! But I won't make an idiot of myself, instead read what great copywriters like David Garfinkel, who is author of Killer Copy Tactics and widely acclaimed as "The World's Greatest Copywriting Coach" says. He uses psychology to great effect: http://www.roibot.com/kc.cgi