Question: In our (web) application, we want tooltips (read: title attributes) that also indicate shortcuts for buttons. We have some button that involve the cursor ('arrow') keys. 

We've tried using characters like ⬅ and ←, but they just don't look right. We've also tried spelling it out, e.g. "Left Arrow Key", but that's a bit verbose.

What's a good way to do this?

Answer: 
Tooltips from the basic 'title' attribute are very limited and not your best option: browsers may or may not pop them up as you expect them.

I think @Rahul comment is very good in this regard: you may want to look at a more complete tooltip solution. There's plenty of pre-made libraries to give you more flexible, solid and appealing tooltips that the basic 'title' attribute.

If you are looking at the title attribute for accessibility reasons (as a fallback option from more complex tooltip solutions), then I think the verbose option ('Press Left Arrow Key to...') is your best option: a screenreader would read it out nicely.
If you move beyond using the title attribute and instead utilize a CSS solution, you could actually provide both a small image of the keyboard key as well as a short description below it. There are a huge collection of very interesting web pages and example images of "CSS Keyboard Key images" use that phrase in a Google Image search for starters here is an erudite page by somebody who cracked the way it is done on Stack Overflow pages see:-

<http://www.jimmyscode.com/css-styling-for-kbd-tags/>