A significant portion of my MBA admissions consulting applicants come to be with little to no extracurricular experience since their undergraduate days. While this is a problem that can be addressed, it can show a lack of proper planning over the long term. A lot of applicants don't think about the impact of their actions on their applicant competitiveness when the graduate from undergrad. To a certain extent, even I was the same way. What I like most about some of my clients is the the way some of them are way ahead of the game we call the MBA application process. Those that have been planning since day 1 to go back to b-school tend to be distinguishing yourself from your peers out of the gate and these habits show when constructing the business school application.

This is what I tell those who have graduated and are planning to apply to business school in a few years:

Make sure you find a mentor at your employer. This does not have to be formal but it always helps when someone can show you the ropes. This person should be senior and be respected in the office.
This mentor will also be able to make introductions for you around the org. This will help when it comes to recommendation time.
Adcoms will consider your tenure and the ratio of leadership experience to overall experience at your work. Work to maximize this so that you can write about it eventually in your essays.
Make sure you are asking for leadership responsibility. It does not matter if it is leading the charge with organizing the company holiday party or asking for more work when on a project. It's called managing your manager and it works wonders when its time for a promotion. Do this and more opportunities will arise. You may get to pick and chose your project work.....
Stepping up also helps with respect to your eventual recommendation when your manager is asked for how well you rank against your peer group.
Seek out those in the organization who have MBAs. Get to know them. Eventually, ask them to lunch and get to know how they liked their MBA experience.
Get involved with extracurriculars at work. Assume a leadership role as well as I have alluded to above.
See what pre-MBA mentoring programs are available to you. I know UCLA Anderson has the Riordan programs. This is a great way to gain exposure.
Join several other organization types:
Toastmasters International - http://en.wikipedia.org/wiki/Toastmasters_International
A young professionals organization in your city - these orgs have social and business elements that will help you network.
Meetup.com - look for groups on meetup that have business or philanthropic goals that align with your interests.
As you get within a year of applying, go to open houses and MBA tours. Get your name and face out there. Adcoms have an uncanny way of remembering who you are.