For example, if your intention is low but you are paying a lot of attention to poor health and feeling emotion surrounding that, the emotion plus the attention overrides the low intention, and you get more of what you are emotionally thinking about. Your thoughts, emotion, and attention perpetuate poor health. If your intention for a new job is high, but you pay it little attention or give it no emotion, chances are it will take a while for that job to manifest - if it does at all. If you intend for that new job and give it attention, but also continue to think about how impossible it is to get a decent job (usually with emotion), then chances are you are negating the intention. Instead, you need to set your intention for the new job, think positively about getting the new job, and catch the negativity before it sets in.
Here's another example. If your intention is for financial abundance and every morning you visualize yourself financially abundant, but every time the subject of spending money or paying bills comes up you think, I can't possibly afford that, you are negating your attention and perpetuating a lack of abundance. In this situation, however, you may notice the thought I can't possibly afford that makes you feel a negative emotion. So if you can't catch the thought, notice the emotion. When you notice the negative emotion, reset your thought and re-visualize your goal of financial abundance. Do this consistently to set the wheels of creation into motion.
So what's the best way to manifest? Combine intention and attention. Set aside time each day to visualize that what you want, and do it with joy. State your intention clearly. Then, direct your thoughts to that intention and combine it with positive emotion.
To avoid unintended creation, throughout the day pay attention to any negative feelings that arise. When those feelings come up, it tells you your attention has slipped away from your intention, and you are creating something you don't intend. Notice those feelings and release them. Then, restate your intention, visualize your desired goal again, add in some positive emotion, and you're headed off in your intended direction instead of working at cross purposes to that which you wish to create.