I'm starting to gather some great souls for enchanting and I plan on making some pretty powerful constant effect enchantments. What's the best way to do this manually?

I've heard there are ways to overpower your character through alchemy, but I've never gotten into alchemy further than it's normal purpose: making spells that I can use in my travels.

So what is the most convenient way to ensure I don't waste a grand soul gem with a rare creature's soul?

Other than that, what are the best clothing items to enchant? And when it comes to weapons, cast when strikes is the best to add damage, so I should use a soul like Vivec or Almalexia, right? Since they're the strongest souls in the game, they'd have the highest charge?