Joey,
The first necessary condition for tight joinery is accurate layout. If the lines are in the right place, it is simply a matter of cutting to the line. A man I really respect said "The score line is neither a suggestion nor an inconvenience."
If your saw is not cutting perfectly, the best thing to do is stay off the line and take it to the line with something you have more control with (as Jim was saying.)
One thing I frequently do with circular saws is watch both the line on the top and the line on the side as the blade enters the wood and you can tweak the saw by slightly raising one side or the other to follow both lines. It takes a little practice, but when cutting joinery on irregular stock, it allows you to cut to the line and create the flat plane you need.
Good luck,
Gabel
