I am a tight knitter too, as you can see in some of my knitting videos HERE
, so one thing that I do almost all the time when casting on is to use a needle several times larger than what I will use for the project just for the cast on row, then I start knitting with the regular size needles. This makes the cast on stitches looser than I can get them to be on my own because the needle is bigger. Another thing I do is what suzeeq said, to cast on over two needles, then just pull one needle out before you start knitting. I do that frequently too and it really helps in getting loose stitches.
Also really pay attention to how tight you are holding the working yarn and how tight you are pulling it once the stitch is formed...the death grip as trvvn5 calls it! Once you figure out how to relax that grip and let the yarn flow, you will really be on your way to enjoying knitting!
Don't give up! It's well worth the effort!