fiction - How much can a reader remember?

I've been writing some fantasy. Now, inevitably because its a big piece of fantasy, its all very involved with the world, culture, story and general bits. Personally, I'm really enjoying writing it.

I have noticed that in a few fantasy novels I have read, I just haven't been able to remember who everyone/what everything was. For example, Thomas Covenant, The Hobbit and Lord of the Rings, Malice represent this perfectly. There are so many materials and things, strange concepts that sometimes I'm completely unable to keep track of them. For example, in The Hobbit I didn't know which dwarf was talking half of the time...

Anyway, in my novel there are many different materials and characters, and the world is really big. How much of this can I expect the reader to remember without having to flick to the glossary or map every two minutes?

Answer

In addition to Mark's excellent advice, I would suggest:

1) Start slowly. In Game of Thrones, we start with just the Starks, and Martin adds on characters a few at a time and lets us live with them for a chapter before bouncing back to someone we already know. Granted that by book 4 you may need to refer back to the index, but that's over thousands of pages.

2) Either build your world or introduce characters, but not both at once. In David and Leigh Eddings's Belgariad and Malloreon pentologies, we start with one boy who has one aunt and one adult friend, the aunt's father shows up, and then the band collects more members one by one as they leave the small farm and go out into the wider world. Each person is distinctive and has a part to play, and the Eddingses either introduce a new setting/city or a new character, but not both at the same time.

3) Remind the reader occasionally. If you want us to remember that Sadi keeps a poisonous snake as a pet, make the snake's entrance memorable, and then remind us every 40 pages or so that the snake is around. Have Sadi feed her, talk about her, let her out of her bottle to crawl around. Then in the big showdown 200 pages later, it won't be a surprise when the snake bites the bad guy, because we've been reminded that the snake exists.