Clash of Clans
If you're winning a battle and haven't deployed all of your troops, don't and keep from having to train replacements when you return home. However, if you do and they survive the battle, all is not lost. They will be converted into elixir you can use to train replacements.
How to get more gems
Obviously, this is how SuperCell makes its money. These are the in-app purchases that keeps this game at the top of the charts. While some people are adamantly opposed to micro-transactions, if this FREE game gives you a reasonable amount of fun, why not give $5 to the developer to keep it going and you get a little boost in the game. Also, if you're one that can't stay away from buying gems, go ahead and make a bigger purchase upfront as the per gem cost is cheaper. Lots of players will buy them $5 at a time instead of just bitting the bullet and getting more value.