For good sfx you really need raw source material to get the physical properties sounding realistic. This can mean recording your own or starting from a sound library. Perhaps part of the problem with making sfx from scratch is that you need a lot of plug-ins to really get the most out of your material, which means spending a huge amount of time (for a pro sound artist this is a life-long career and many thousands of dollars) getting to know sound properties and how they are affected by signal processing. The big game companies go to pro sfx houses who have massive libraries to work with and dedicated recording equipment and foley rooms. The big studios, for example, have foley specialists whose techniques are top-secret to her and her team, and their audio is then passed on to another team of specialists for treatment.

For a really good read on soundfx see the Star Wars book:

http://www.amazon.com/Sounds-Star-Wars-J-Rinzler/dp/08118754...

I can't recommend this book highly enough for an engaging history and deeper look into sfx, it comes with a bank of sounds for you to listen to from the film, as you read.

For an unexhaustive selection of raw material see http://freesound.org (don't forget to attribute!).