What can I do about neighbors parking on my driveway and sidewalk?

I suggest having your property manager or attorney send a letter by certified mail reminding the apartment manager of the actual rules, and informing them that on X date, the rules will be enforced by towing/issuing parking tickets. (Consult and attorney to determine which you can actually do.) It can't hurt to post signs stating where parking is prohibited and the consequences of violating the rules. The signs bypass the 'I thought it was legal' defense. While ignorance of the law is not a legal defense, knowledge of the law makes things easier for the enforcer.