Pelvic organ prolapse may have a genetic component; if one or more of your close female family members (mother/aunt) have experienced this, you may be faced with it as well. This should not stop you from reducing your risk however, remember that an ounce of prevention is surely better than a pound of cure.
For starters, beginning Pelvic Floor Muscle Exercises as early as possible is always a good idea. This will keep your muscles which support the bladder, uterus and rectum as toned as possible. Since 95% of the supports of these organs are the muscles of the pelvic floor this makes sense (only 5% is connective tissue which anchors these organs to the muscles, and these muscles to the bony pelvis).
To start, identify these muscles by tightening the vagina (same muscle as when you try and stop your urine flow). Once you've identified this you don't need to actually do these exercises while you're urinating since this is not especially helpful and may lead to bladder infections.
Hold this muscle for a count of 5 (5 'Mississippi), relax to a count of 10 and repeat this 5 times (a 'set'). Try and do up to 10 sets per day, then build to 20. You may notice a difference, especially if you've experienced a bit of prolapse before initiating this program. It takes about 6 weeks to actually notice a difference, so don't be discouraged. Your doctor can also prescribe pelvic floor physical therapy for you; there are even vaginal weights or cones to help motivate you!
Of course, not smoking or quitting smoking is a good idea for so many health reasons (for you and your family); the chronic cough which usually results plays havoc with your pelvic muscles and really stresses and weakens them.
Other good tips include keeping as optimal a weight as possible (this reduces the stress on your pelvic muscles from above-reduced pressure from your abdominal cavitiy), hydrating enough to avoid constipation (pushing stool out also weakens the pelvic floor), and avoiding repetative heavy lifting (same reason as above).