The human body is less tolerant of a negative G's which force blood into the head. Large amounts of sustained negative G's can result in uncomfortable symptoms such as facial pain and readout. Although some experienced aerobatics pilots maybe able to withstand 7 or 8 positive G's before blackout occurs, most will be incapacitated by only -3G's. You can improve your G-tolerance by maintaining good physical conditioning and avoiding smoking, hyperventilation, and hypoxia. Most civil pilot however, will not encounter G-force of sufficient strength during the normal flight to cause any major problems.
