I started to post this to Dara in the comments, but then it got a mile long and turned into a whole new post.
I'm not much of a goals person. If I set any sort of a numbers or performance goal, the obsessive part of my little brain will latch on it and not let go until I've driven myself and everyone around me insane. For example during Pull-Up Quest '06 I was doing pull-ups on a wet metal jungle gym on a hill during a thunderstorm. Once I set a goal, I would literally rather get hit by lightning than fail.
I haven't figured out the secret to balanced, healthy goal setting. For me, the only way to keep things even remotely in perspective is not to set anything too specific. I can have the general idea that I'd like to build muscle now, or like to get a little leaner now. But if I specify a number and set a deadline, I turn into a lunatic.
Back in days of yore, I used to photoshop my head onto fitness competitors' bodies. I got pretty good at it, so good that people would send me their own photos for "head transplants." That way, even remedial visualizers could see themselves with the body they wanted. They could put it on the refrigerator, use it as desktop wallpaper, make the image more of a reality every day. It was powerful stuff. In many cases, people became even more fit than the body they'd chosen!
Now, if I visualize anything health and fitness related, it's just myself as a fit person doing fit things. When I start my day, I see myself working out, eating a healthy breakfast, packing my healthy meals for work, having fun, feeling confident, enjoying myself. I think a lot of people screw up because they picture themselves eating the leftover cake in the kitchen, and how good the cake will taste, and what they could have with the cake, and when they'll be able to eat the cake uninterrupted. You can use your visualization powers for good or evil.
It helps to remember that anytime you worry, you're visualizing. You're taking the worst possible scenario and playing it over and over again in your mind. You're giving powerful emotion and energy to what you DON'T want. Most of the time it's a totally subconscious thing. You can do it for hours and never realize it. If you learn to pay attention to your thoughts and direct them in a positive way, that's a huge step toward achieving your fitness goals or anything else.