The language on this Cookie Law opt in slider is a great example of how to phrase things.

1. It isn't alarming, it doesn't introduce fear, uncertainty and doubt
2. The default option is a "No, thanks" -- which is feels like "go away and stop bothering me with this irrelevant thing I don't want to do now..."
3. It stresses "best experience"
4. The other option is "change settings" -- aka "fiddle around with stuff I don't normally do"

It's a great starting point and I imagine we will see lots of clones.
Great example of UK Cookie Law Opt-in Overlay from BT
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