This is a very common question and one that has a simple, yet complex answer. Most of us do not consciously create our own experiences at all times, in fact most people run on "autopilot" directing most of their energy to wherever their minds go and whatever they perceive to be experiencing. A mind left to its own devices can be a dangerous thing. Most of us let our minds control us - rather than learning to control the mind and that leads to a lot more struggle in life.
Most people will say "I would never create illness, poverty, etc." and my response to that is yes - you might. No one likes to hear that. No one wants to believe that their thoughts/actions can attract "bad" things. The truth of the matter though is that where thoughts go energy flows. Also important to note is that all experience is necessary - learn that illness makes you appreciate health, lean times make you appreciate abundance etc. If you focus on turning your challenges into stepping stones rather than stumbling blocks you'll find the Universe truly opens up to you.
If you are ill - chances are you spend a lot of energy thinking about how bad you feel, how much you hurt etc. If you have become ill the best way to heal and work with your body is not to focus on how bad you feel, but focus on how happy you are that your body is working to heal. Focus on gratitude for the chance to rest. Focus on anything positive and not on "woe is me"... If you focus on pain, discomfort, illness etc. it will continue to manifest. Can you speed healing by redirecting your energy? Yes when applied properly. This is the force in play with many "unexplainable" medical miracles.
Here is another example, some scientific studies on prayer showed that ill people who had others praying for them - whether they were believers or not and without knowing they were being prayed for - still healed faster than others. Why? Because positive energy and healing manifesting energy was being directed towards that person and the individual was open to receiving that energy. So it follows then, if you want to heal, you need to direct positive energy towards your healthy body as you work with your doctor of course ;). Say thank you for healing body! Express gratitude for the opportunity to better appreciate a healthy body. Meditate and visualize the healing light moving through you and all around you.
To maintain good health, take time to appreciate your body and all it goes through to sustain you every day. The body is an AMAZING piece of machinery and face it - most of us severely neglect, abuse, and take it for granted. I highly recommend visiting a "Body worlds" exhibit if one ever comes to your area - WOW.
Another common example of a negative people don't realize they attract is debt/lack of money. People tend to focus a great deal of their energy onto lacking money or fearing the loss of money. What many fail to realize is A) What you focus on manifests and B) What you resist will persist.
Many people tie some of their greatest fears onto money and what they are really afraid of is not "money" but loss of security, feeling powerless, what others will think of them etc. SO many fears are attached to "money" and fear is a very powerful energy - if you live your daily life chronically afraid of loss, expecting that you are limited to your current job/income, and feeling overwhelmed by debt, ... well guess what is going to continue to manifest in your life? Not only are you creating what you expect to see - you are resisting what you are afraid of - double whammy!
Want more money? Work on managing (not eliminating) your fears and FOCUS on abundance... focus on giving to others, focus on gratitude, focus on the opposite end of the spectrum - instead of fear, loss, lack etc. direct your energy towards prosperity, fulfillment, freedom, etc.
See also "What are you afraid of?" & "What do you really want"?