What's the proper way to punctuate this?

I wouldn't use the comma, but this is casual usage, and the formal statement would be more complete.

BrightSera said:

I am at the point where I fear commas. *paranoia*

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If commas are really after you, it's not paranoia.

It's always "proper" to set off trailing adverbs or adverb phrases when the stand in apposition. If you set off these adverbs:

Are you going, too?
Oh, yeah? Well, he spit, also.
She's sweet, however.
So you'll marry me, then?

You should be consistent and set off either: It wasn't my dog, either.

Lots of modern writers don't set off adverbs, so:

Are you going too?
Oh, yeah? Well, he spit also.
She's sweet however.
So you'll marry me then?
It wasn't my dog either.

Especially in fiction, the choice seems to be yours.

Use the comma.

Jamesaritchie said:

Use the comma.

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My preference, too.

Sometimes coma are also a matter of rythm. If you want to break your sentence and make it slower to read, put the coma, otherwise, don't.

With questions like this, I like to go to the Chicago Manual of Style Q & A. I sometimes peruse this section just for fun. Which says a little too much about me probably. Anyway, they have this to say about commas in such a situation. (This is about 'too' but I think it still applies.) http://www.chicagomanualofstyle.org/qanda/data/faq/topics/Commas/faq0004.html

salome said:

With questions like this, I like to go to the Chicago Manual of Style Q & A. I sometimes peruse this section just for fun. Which says a little too much about me probably. Anyway, they have this to say about commas in such a situation. (This is about 'too' but I think it still applies.) http://www.chicagomanualofstyle.org/qanda/data/faq/topics/Commas/faq0004.html

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That's pretty much what I thought also. (no comma before "also")