As others here have suggested, successful processing is about controlling variables, and altering only the ones you want to alter. Simplest is usually best. You have to think of everything that happens from the moment of exposure to the final print as interlocking steps in a "system"; changing one thing often changes others.
Look at the shadow areas of the negative to evaluate your exposure. (These are thin or transparent in the negative of course.) If your negatives have adequate detail in shadow areas that you did not intend to render as featureless pitch black, then you have adequate exposure (ie, proper EI for the film/developer combo you're using.) If areas that should be really black in the final image are more gray, then you've likely overexposed the film and should shoot at a higher EI. "EI" of course is your personal "adjusted" speed rating for the film/dev combo in question; this value encompasses a lot of stuff, such as the type of metering and your methodology in using it; the accuracy of your shutter; and the ability of a given developer to give full "box" speed with a given film.
To evaluate development, look at your highlights, those dark dense parts of the negative that correspond to lighter tones in the final print. If your negatives have very dense highlight areas (dark in the negative, some shade of white in the final print) then you've probably overdeveloped. Do you have detail in highlights where you intended to have it? If not, you've "blown" them and should reduce development time, while keeping temp and agitation as they were. It is possible to be both underexposed and overdeveloped, which is probably the worst error to make, unless you're pushing your film, in which case it's intentional!
If you've overdeveloped, try a 15% reduction in development time and see what that gets you, while keeping temp and agitation unchanged. Temp control is probably the place most people go wrong--as someone said, 1 or 2 degrees can make a difference.
