Looking at already existing addons is what helps me a lot
After I know what commands are used I look them up at
http://www.wowwiki.com/World_of_Warcraft_API
Very well documented Frameworks are a good help, too.
The wowace-community is probably the best source for documented
wisdom and experience in addon writing.
In the beginning the ACE3-Framework
http://www.wowace.com/addons/ace3/pa...tting-started/ shows what is possible and what is needed
in an addon.
Last but not least WoWInterface-Forum is full of helpful and
friendly programmers who tried everything you dream of already
and share their gathered knowledge happily
Edit: A good start to see what problems and solutions are found with unit frames search this
forum for posts of Grimsin ... his threads have really useful and interesting info in regard of unit frames.