These are mostly out of your hands - or are they?
There are a couple things you can do to get the most out of these letters:
If you're conducting a purely non-academic job search - as I was - ask your advisers to signal that in the letter. I've seen firms pass over strong candidates when we feel they're primarily interested in the academic market; our belief is that they'll end up accepting an academic offer over ours. So when we see evidence to the contrary in letters of recommendation, if increases the likelihood we interview them.
Ask your advisers to mention specific skills or interests of yours in the letter. Ideally, if these interest are skills are positive signals to industry (or other employers), you're mentioning them in your own CV, Resume, or Cover Letter. However, your advisers have the chance to function as additional positive signals on this front.
If you feel you have a good relationship with your adviser, you can ask to read the letter. If you don't - probably don't ask them for a letter. I know this is a little uncomfortable and I'll confess to not doing this, but at least one of my advisers just showed me the letter he was writing and asked me what I wanted him to change.
Like cover letters, letters of recommendation have a very non-linear impact. Many times, they go unread. However, when they are read, they can make a difference. Given that it will take you an hour or less to send your advisers a short (1 page) document listing the skills/items you'd like them to mention and the job-search focus, it's worth your time.