One thing I realized about writing maintainable code applies just as well to designing accessible interfaces: the right habits get you 80% of the way, with no real cost beyond developing the habits. Getting into the right habits is an O(1) investment that pays off by making everything you produce more accessible.

Adding alt tags to images, using high contrast colors for text, writing clean, well-structured markup... all these things don't take significant time on top of your normal work as long as you do them right away. They also have additional benefits like making the site easier to parse for search engines.