Some of the members of this forum could probably write a book on how to play Dominions...and its one of the few games that would probably warrant it. Maybe a 17 volume set...one for each nation!
As to shields, I believe they greatly increase the chance of blocking arrows. Maybe all missiles, but they don't seem quite as effective vs. javelins, so maybe someone else knows?
Magic searching is an art I don't claim to know everything about, but using the spells is a decent method. There are spells, not too far up the research tree, that will find all magic sites of a certain element in a province, and one that will find them all, but it is very expensive to cast. Additionally, some players like to take a "rainbow" pretender with 2-3 magic in each path and then use him/her to search provinces. Because no site requires more than 4 of a path to find (and those that do are rare), such pretenders will find almost every site they search for. Finally, if you look at the scales of a province you have strong dominion in and they are still not at the right levels, that is good evidence of a hidden site. Additionally, unwarranted unrest can be a sign.
This game is so incredibly deep, just asking for all the tips and tricks on how to play it probably won't get you too far, unless you have specific questions. I'd recommend reading through as many threads as you can stand on the forums here, as there is a dearth of excellent information to be had. Playing the game a lot never hurts either

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