Into
Use into if you are describing something in motion or something completely entranced with something else:

• She walked into the shoe store.
• The kids jumped into the piles of leaves.
• He is turning into a werewolf.
• They were totally into Jim Morrison’s poetry.

The muddy boys are jumping into the lake (photo: http://www.flickr.com/photos/laffy4k/247066114)

In to
Use in to if in is used as an adverb and to is used as a preposition connecting the verb to an indirect object.

Think of it this way: If the sentence could technically end after in, then you can add a prepositional phrase (i.e., a non-necessary phrase that starts with a preposition and adds a bit of detail to the sentence) by using to after in (but not changing in to into.

Clear as mud? Here are some examples:

• The concerned citizen turned the wallet in to the police. (It could easily read The concerned citizen turned the wallet in. The prepositional phrase is to the police — interesting additional info but not completely necessary for the completion of the sentence.)
• I will not give in to chocolate cravings.
• He was nervous about handing his assignment in to the instructor.

Pooch gave in to the power of nap time (photo: http://www.flickr.com/photos/nesster/3701101878)

See? I told you it was a breeze.

Happy trails!

SAK