Ghosting your troops is an essential tactic that everyone must learn in order to excel in both Battlefield and SvS. The basic principle is to make your troops invulnerable even though they are technically still in your city.
Ghosting is achieved by putting your troops in a 'fake' hour long rally. You just set rallies with your max march size or whatever troops you want safe on another persons city or in a pinch ghosting on a boss monster will work. When going to the enemy server during SvS it's a good idea to have your ghosting target already picked out so you are not frantically looking for one once you're there.
Ghosting is great because you can simply cancel the rallies and have your troops home to take a hit or to move. You can ghost part of your defending force and wait for someone to scout you and then cancel the rally so that they run into more troops than they were expecting.