There are two steps to deciding if you should be taking aspirin for prevention of heart attack (do not follow any of this advice without talking with your physician first).  The following guidelines are from the New England Journal of Medicine, May 9, 2002 (volume 346, No. 19).  First determine your 10 year risk of having a heart attack.  To do this, fill out the simple form at the following link:  10 year risk of heart attack.  

• Now, if your risk of heart attack is less than or equal to 0.6% per year then you should not take aspirin to prevent heart attack.  
• If your risk of heart attack is 0.7 to 1.4% and you are in poor physical condition or have treated hypertension and either diabetes or target-organ damage (damage to the kidney's, eyes, or brain), then you should take aspirin.
• If your risk of heart attack is greater than 1.5% then you should take aspirin.

The dose should be a baby aspirin (less than 100mg per day).