spelling - "Its" and "it's" - how to remember?

What is a good mnemonic rule that an English learner can use to remember the difference between its (possessive adjective: a team has started its lunch) and it's (verb: it's raining)?

Answer

I'm not sure of how good a mnemonic this is, but it's always worked for me. When proofreading, I simply remind myself:

You can't remove an apostrophe when it's taking the place of a letter.

So, when I say:

It's supposed to rain tomorrow.

I recognize that "It's" means "It is", so I leave the apostrophe in. But when I type and proofread:

The house is losing it's foundation. [sic]

I realize that "it's" does not mean "it is"; it means "the foundation belonging to it", so I remove the apostrophe:

The house is losing its foundation.

If you don't like my method, it's easy to find lots more on the subject.