As both a sound designer and mixer, I'll put a few bugs in your ear, none which are "magic".
First, be careful with processing if you are NOT the mixer... something that sounds amazing in your edit suite can fall apart very quickly on the dub stage. Always consider the greater mix when your building sounds. The most amazing sound by itself, is not always the most amazing in a mix. Also if you're finding that your sounds are great in your room, but not at the dub stage, then consider looking at your monitor chain (and all that entails).
If you are both the mixer and the designer (as is very likely these days), and you're happy with the sounds themselves, then it boils down to the mix. Having spent the last 10 years mixing a LOT of shows, I can say that 90% of battle is level, the other 10% being processing (EQ, Verbs, etc). Making sound effects "real" and sound like they belong in the track rather than added on is an art that takes a lot of time to master. My best education came from sitting with some experienced mixers on my early stuff and watching how they dealt with things.
Finally, always be aware of the "I suck" phenomenon. Not that you ACTUALLY suck, but it's easy to compare your work to someone else (especially people you admire) and find it lacking, when everyone actually thinks it's good. I remember talking to a fellow mixer and commenting that I always found his mixes so much better than mine, and he turned to me and said "Funny, I always think YOURS sound so much better". Sometimes getting an unbiased opinion shows helps a lot.