Over 40% of heating is lost through your ceiling so it's really important to make sure you get rid of any draughts or areas of air leakage. Check around all light shades, vents and antennas for potential gaps. Crawl into your roof space and if you can see any light, that's a gap that's causing heat loss and needs to be filled.
Your house can lose a lot of heat through a chimney or open fireplace. If you're not actively using your fireplace, consider sealing it up on the inside. If you do like to use your fireplace, you could close the chimney's flue or use a damper to reduce the air flow.
Air conditioning units can lose a lot of air at the connections that lead outside. Check that all the fixings are secure at the building and at the air conditioning unit. Also look for leaky or cracked pipes that may also be causing air loss.
To seal aluminium doors and windows, install a thermal break to create another barrier against air loss and movement. For wooden windows and doors, you can use foam seals. Make sure that any existing door and windows strips are in good condition and that they aren't leaking air. If they do, these can be easily replaced with foam strips.