Many schools blatantly state that clinical exposure is required for admittance (NU comes to mind). Definitely volunteer/work in a hospital (clinical) setting before applying.
One thing though...before picking a place/activity to devote your time, think carefully about your motivations for entering medical school. You really want to create a unified theme with your PS, your LORs, and your volunteer experiences. For example, if your motivation to enter medicine is to serve the poor, choose to volunteer in a hospital that serves the poor; not some shielded hospital in the 'burbs. This way, you can easily tie your experiences with your motivations.
...just my 2 pennies.