ID, id, or Id?

Id is an abbreviation, not an acronym, so I case it "Id." UI is an acronym, and acronyms--short ones, anyway--get capitalized: "UI."

I do what I feel like. In general, your best practice is to err in favor of readability vs. compliance with some abstract standard.

Just be consistent.

Here's a sample of what I do:

id
userId
getUserId

The key is being consistent.