Invincibility:
Press V to become invincible to enemy fire or damage. Note: This cannot be enabled in campaign mode. 

Edit mission scripts:
The default mission scripts can be declassified. Enter the mission editor and try to access the pull-down menu. Leave the password prompt blank to declassify a mission. Once this is done, it may be edited to allow invincibility (under the "Skills" setting), hard landings, recovery, and more.

Hint 1:
Although not a documented feature, pressing the V key will make your plane invulnerable to damage or enemy fire. Unfortunately, this cheat won't work in the sim's campaign missions.

Hint 2:
Each of the default scripted missions can be "declassified" at the Mission Editor screen's pull-down menu. Although a password is asked for, you can leave this blank, because SSI has applied a null password setting here. Once the mission is declassified, you can edit it to your heart's content. (Tinkering with the Skills settings, for example, will allow you to enact the V key cheat and other newbie aids, such as "Hard Landings" and "Allow Recovery.") The campaign missions are still classified, but SSI plans to release the appropriate password for them at an unspecified future date.

Hint 3:
When linking up with other players for a cooperative multiplayer match, you can get a clearer view of each other's positions by turning on wingtip smoke with the T key.