The key to a profitable list is to not constantly bombard your subscribers with sales pitches for products you are selling or promoting, but to offer plenty of good quality content as well. If you don't do this, you will see a lot of people unsubscribing from your list, and all you'll have is a list of unresponsive subscribers. It's also important to get the email frequency right.
You should strive to provide a balance between email list too many and too few emails. Too many and people will become fed up with you and will unsubscribe from your list. Too few and people will forget about you and will not readily recognise your name so will be more unlikely to open your emails in the first place. In my experience one or two emails a week is about right.
So to sum up, if you want to increase your revenue from your websites, you have to build a list. You need not create one for every website you build as this can be very time-consuming, but you should look to build at least one main list that you can generate good profits from. The major step is to sign up for an autoresponder as this will allow you to both build your list through an opt-in form, and send emails to this list.