This is an easy one: Don't bother entering the exact feed URL when subscribing!
These days it is standard for websites to support feed auto discovery. Many blogs and websites actually do not add feed subscription icons and links anymore. The feed link is just included in the HTML markup and every aggregator can extract it. No need for you to look for it.
To make it even simpler combine with trick #2 and use drag&drop! On every websites whose feed you want to add just drag the favicon from the browser location bar into Liferea. Most browsers do interpret this as dragging the location. Do not even bother copying the URL.
Be lazy :-)