Your book should have a message too, but unless your audience is 2, you're gonna lose them if you're more message driven than story driven.
So how do you get your point across without lecturing?
Trust your reader to get it.
One of the trademarks of lousy books is when an author says something multiple times and leaves you feeling like, "Yeah, I get it. You've said it 3 times in this chapter alone." So say your theme is trust. You don't need to point out your character's trust issues every time they come up. Actually, if you're doing your job well, you shouldn't ever have to state, "Maggie has trust issues." (See the post on Showing vs. Telling for clarification on this.)
Give us a Reason
Think about the issues you have. You have them for a reason, right? I didn't pop out of the womb with a hatred for public bathrooms. That hatred is linked to something specific - a faulty lock on a bathroom door at a bay area Starbucks. Make sure you're weaving your theme into your character's background.
Make it Matter
I read a book last year that not only lectured like crazy, but it also had about 50 social issues it was attempting to address within the same story. Everything from HIV to ethical journalism to the education system here in America. And I didn't really see how any of it was relevant to the actual story. So make sure your theme actually matches your story. And that you don't have a thousand points you're trying to make. Otherwise your readers (or at least this reader) will wind up chucking your book across the room.
Say your message is that divorce is wrong. Maggie doesn't want to be married to her husband anymore, but she feels divorce is wrong, and so she's staying in her marriage. To avoid lecturing your reader, you're going to need to show us a little equality. We need to see why divorce could work out to her benefit. Maybe you've shown us examples of couples who stuck with their marriage and 30 years later are happier than ever. Now show us a couple who stuck with it and are now miserable bags of bones. If you showed couples who divorced and are upset with themselves for it, now show us couples who divorced and feel it was the best decision they ever made. That not only keeps you from lecturing, it makes your story unpredictable.
Stories need themes. They enrich your plot and your characters. And sometimes they even enrich your readers' lives.