What should I look for in a sunscreen to prevent breakouts?

Non comedogenic. Do not use oil base or products that clogs pores. Look for term non comedogenic.
Sunscreens & Acne. If you have acne/ break-out prone skin, look for sunscreen formulations that have been designed for acne-prone skin. They should be labeled oil-free, non-comedogenic or both. Broad spectrum uva/uvb protection is critical. A very nice option is a highly refined mineral formulation that works well for all skin types and can be effortlessly reapplied all day long (over makeup) without clogging pores.