Well Xonathan snuck in and stole some of my answers but here's my analysis...
More important perhaps than what is in your Reserve is your ability to get cards out of it. If I can, I like to have no more than 3 cards in the Reserve that I can't get out using any other card besides Covenant of Prayer or Gain LS. If I do end up with more than 3, then I definitely want to be running Gain LS and Imitate LS so I have a better chance at getting a couple extra cards out of the Reserve.
Obviously The Deceiver should be in any defense that is mostly humans so putting 2-3 human evil characters in the Reserve makes a lot of sense. Characters like Foreign Wives, Gomer and Goliath can catch your opponent off guard if you pull them from Reserve.
I don't like putting many artifacts in the Reserve unless I'm running Heavenly Temple Artifacts with Servant Angel. The problem with Artifacts is that if you need it right away, you're going to be stuck grabbing it with Covenant of Prayer, which means you already used your artifact slot that turn--so the earliest you can activate is the next turn. On the other hand, NT Covenants can be grabbed with 5th Seal so those are a bit better to have in Reserve. I agree that Three Nails is a solid choice for the Reserve, but I would probably make sure I was running Gain LS so I wouldn't be forced to grab it with Covenant of Prayer.
I also like putting situationally strong territory class enhancements or a spare Hero in the Reserve so I can use them right away if I have to grab them with Covenant with Prayer. This allows me to keep my battle winners in the main deck.
Lastly, if you do run Gain LS, I believe its good to have a plan going in for the first card you shuffle into your deck. Basically, think of what card you would add to the main deck if you were running one more card and make that your first target for Gain. Obviously that can change (i.e. you see your opponent has a demon defense so you might shuffle Three Nails from Reserve as soon as possible), but try to have a card or two that will always be useful, but that aren't necessarily needed in the early game. I have found cards like Paying Taxes and Treasures of War to be such cards.
