At the Title screen type DIRK.
You have to have a sector editor of some sort i.e. Norton's DE or the like. First edit the save file if you don't have one all you have to do is press 's' on any screen in the game. Then for unlimited lives change offset 4 to 99. Level jumping is also quite easy... all you have to do is add 5 to the level that you want to go to and enter that number in offset 1.