Draughts from around your doors and windows can account for a quarter of your heating and cooling bills. Exterior doors in particular can contribute significantly to air leaks, and can also waste energy through conducting - especially if they're old, uninsulated, or improperly installed and sealed.
Draught-proofing can reduce the energy losses due to air leaks.
Door snakes are a cheap and easy option.
Weather strips only cost a few dollars per roll and stop air coming in or leaking out.
Do not attach anything to fire-rated doors in your apartment e.g. the main entry/exit doors. Speak to your landlord or owners corporation about what permanent solutions can be applied to such doors.