1. Apply earlier (avoid the last six weeks before the deadline).
2. Apply to more schools (six is usually considered a prudent minimum: two safe schools, two middle of the road schools, two reach schools).
3. Apply to more safe schools (even 4.0 students can and do get rejected).
4. Visit and wow 'em.
5. Go to summer school in the targeted subject and wow 'em (it's easy to get into summer school, even at Harvard).
6. Take one class at a time in the targeted subject and wow 'em (remember: your most recent grades count the most).
7. Get volunteer or internship experiences in the targeted field (even part-time, even unpaid).
8. Work in a "real job" in the targeted field (there's no substitute for actual experience, and recommendations from supervisors in the profession).
9. Get an intermediate degree (such as a Masters or even just a credential).
10. Get older and try again (many times, that's all it takes).
Adapted from Asher, Donald. Graduate Admissions Essays. Berkeley, CA: Ten Speed Press, 2008.