All man coverages are not created equal. There are two very commonly used versions of man coverage - one with a safety valve, and one without.
When running a full man coverage, or Cover 0, every eligible receiver is covered man to man by a player on the defense. That means 5 players are in man-to-man coverage. The other 6 players are rushing the passer or playing the run, depending on the offensive play. This is probably the most basic form of defense there is, but if you don't have the athletes to match up one-on-one with your opponent, you could be in for a long night.
Man Free Coverage, or Cover 1, includes a 'safety valve.' One player is assigned to cover 'deep,' though his technique will depend on the coach and the style that he would like the free safety to play. You may allow the free safety to focus in on the run first, and then help where he can. More commonly, the safety back-pedals on the snap reading the Quarterback. If the Quarterback turns his shoulders in one direction, or rolls in a direction, the Deep Middle player is now looking to help on coverage of a receiver to that side.
With Man-to-Man coverage, it is important to realize that your next best help against a pass is going to be the sideline. Therefore, it is most important to align inside of the receiver and force the Quarterback to throw the ball over the defender, and inside of the sideline.
On the other hand, with Man-Free coverage, the defender may have help from a Safety. Often by alignment and field position we can know where that help is most likely to come. If a defender is isolated on the wide side of the field with a single receiver, he is unlikely to get much help from the Free Safety. Therefore, he should align inside and use the sideline as his help. But on the short side of the field, or with a slot receiver, the defender should be able to expect safety help. The Free Safety is more athletic than the sideline (we hope!) so align outside and know that if the ball is thrown to the inside, the Free Safety will be there for help.
It is important to understand the distinction between the coverages. A third, common man-to-man coverage is the Man-Under Cover 2 defense. This is really a passing downs only coverage for most teams, because of the limited number of players available to play the run. With 2 players playing deep-half field coverage, and up to 5 players matched up in Man coverage, only 4 players are available to stop the run versus 5 offensive linemen!
Use man coverage wisely, but don't be afraid of it just because your athletes are 'average'! Mix man coverage into your cover package to give a change-up to the offense.