Joining affiliate programs is free and easy. So much so that affiliates tend to do it in 'bulk' and pick and choose later. Reading this
affiliate joining tip article, I agree that an affiliate's attitude towards affiliate marketing is highly reflected in the manner of which the affiliate join programs. Treating each affiliate program that you join as an individual business that will require your focus, time and effort to build and work hard on will get you much better results. The thing is, there's a 'development' ceiling that needs to be breached before a program starts producing consistent and satisfactory income for you. Promoting 100 programs is gonna divide or even discourage you from putting much effort into it and all of them will be underdeveloped, possibly failing miserably.
Putting banners on a site is but one of a hundred things you gotta do for each program. Remember, if you only do 2-3 tasks for each program, you're not doing enough.