What is the hardest part of being a fantasy/sci fi author? To me, and this is my opinion only, I think the hardest part is building the world you want your characters to inhabit. There is so much detail to work out, and you never put all of that detail into your books. At least, I would hope you didn't put all of that into your books.
The most comprehensive version of world building put into a print form comes from David and Leigh Eddings in the publication of their The Rivan Codex. Into this book they put all the details they came up with for the world that the characters of the Belgariad and the Mallorean series inhabited. Not all the details were used in the books, but enough of them were there to establish that this was a populated world and each race was unique.
The grandest scope of world building, from what I've read, is a toss up between J.R.R. Tolkien and his Middle Earth and George R.R. Martin and his Westeros. The details needed to create the rich tapestries that make up those worlds is astounding.
I'm discovering this as I work to build Aleran. There are so many details to work out, details that will probably never end up in the story but are still essential for me to know. Races, lands, laws, coinage, mannerisms, costumes. All of these things need to be worked out. Thankfully I'm able to do so with a little help from Himself, who has built many worlds for his D&D campaigns.
What world building tips do you have? Mine is keep a spreadsheet of ideas that you're using. The more you expand on them, the better you'll do.