The various people looking at your application may differ in how they weigh its various components. Therefore, you should pay close attention to each one of them. At the same time, keep in mind that few applicants achieve excellence on all of the important dimensions, so you should not be discouraged if you think you come up a bit short on one or two of them.
Some shortcomings are easier to overcome than others. For example, if you lack work experience or research experience in your field of interest, you can probably fix that. Start thinking about volunteering in an area that is comparable to the graduate program and eventual career that you plan to have. For example, if you are planning on a career as a social worker or counselor, then you may want to consider volunteering in a women’s shelter or veteran’s hospital. If you are thinking of applying to law school, you should consider approaching a law-related organization or community center. If you are thinking of a graduate program in the sciences, consider a research lab at a university or hospital.
Remember that the point of getting volunteer experience is to be exposed to potential mentors and/or experienced professionals that can assess your skills, eagerness to learn, level of maturity etc… and that can eventually provide you with effective letters of recommendation for your grad school applications. This type of relationship does not culminate in a few weeks. Instead, you will have to dedicate many hours over the course of several months or years, so plan ahead. Once you have accepted a volunteer position, be proactive about how you spend your time there and make your goals clear to those who will be evaluating you. Otherwise, you may end up doing clerical or data-entry work that may not be especially beneficial to your long-term goals.
Some shortcomings that may be more difficult to overcome
If you have already written your standardized tests and obtained poor scores, there is nothing you can do to erase them completely from your record, but if you have the time and money, you may want to consider retaking the tests at a later date. Likewise, there is little you can do to hide poor grades that appear on your transcripts. But you should consider retaking a class where you obtained a poor grade, if it is relevant to your long-term objectives. So for example, retaking a Psychology course on cognitive development may not be necessary if you are considering law or med school, but may be worthwhile to retake if you are considering psychotherapy or family counseling.
Preparing an effective personal statement (also referred to as a statement of purpose), is another way to overcome shortcomings in your grad-school applications. The personal statement provides you the opportunity to explain some of the shortcomings that may exist in your application, such as a period of poor grades, or lack of research publications. Be careful however, to not simply blame others for your limitations, as this will most certainly be looked upon unfavorably, and may also ruin your chances of getting into graduate school. For more advice on writing a strong personal statement, check out part 1 of a 5 part series on writing personal statements
As you will see, there are a multitude of other things that you can do to make yourself an attractive candidate. Do not worry about things you can do nothing about. Optimize what you can — it might make up for any irreversible weaknesses.