[@Cynder] I've never had to ask Óðinn for help yet. Bravery and strength are things that are, quite literally, #1 on Óðinn's list of values you need to have. I can't be brave if I always ask him for stuff, and I can't show strength if I need his help in every battle.

Typically a variety of things were offered, mostly food. Eh, that means animals blood, animals meat, or live animals being slaughtered and offered to him at a runestone or holy place.

There's some accounts that also detail human sacrifice. .. varying accounts agree or disagree with these accounts. I honestly think it's very realistic that human sacrifices were real. Enemy soldiers were often captured as were slaves. I think the most favorable sacrifices were enemies of the Norse religion - so Christian lords that wanted to kill all the pagans and conquer the lands.

Then again it's important to notice that the Norse people were way ahead of most medieval societies. Slaves had rights (they were pretty much equal to the serfs of medieval European countries) and were allowed to privately own items, own money, and buy their freedom. Most slaves were treated well. Masters weren't allowed to beat their slaves for no reason. 

Fun note also was that the Vikings dabbled in hygiene long before the more Southern medieval people did. :lol