secure & non-secure items in customer checkout

We have had a few customers who have been perturbed by the browser message "This site contains secure and non-secure items.  Display non-secure items?".

I realize that this is a standard message generated by the browser and I know to simply click "yes" instinctively if it's a site that I trust, but people who are not very familiar with online shopping can be a little shaken by it... especially with all the recent talk about identity theft and so on.

Is there any way to help minimize how many browsers display this in response to my checkout pages? 

I realize that customer's browser security settings, etc. can impact this, but is there something I can do to prevent it? Are there certain html/CSS tags that might trigger this that I could scan for and eliminate on my page?