Lots of simple things I can see I do so wrong. Using headings is a great start to allow users of text readers to find their way around a lot of text.
High contrast text and using HTML or word documents instead of pdf is important. Also the importance of creating meaningful text for hyperlinks (which should also be underlined) and not just putting "click here" repeatedly.
Alternative text for illustrations is something I confess to being too lazy to do consistently. Having a presenter with a visual impairment to explain the implications of NOT thinking about accessibility was really helpful and illuminating in a way that just reading the manuals isn't, and she did this with such humour and good grace that noone was made to feel bad for all their previous errors (OK I mean me...)