I try to wait until a cold windy day and then I feel around for drafts to see where the cold air is coming in.
I think my windows would be called double hung and when the wind blows I can feel a draft blow up between the two windows. To seal up that gap I used D channel and rather than split the piece, I folded it in half and pressed it into the gap. It is a little longer than the window so if I need to open the window the piece can be removed and then placed back in.
I bought a roll of rope caulking and added it anywhere along the window that I felt a draft. It can be removed later if need be.
I also used the outlet sealers. I am not sure if I have a lot of heat loss there but figured it was worth the few dollars.
We used to have a lot of air come through our front door. This year we finally figured out a solution. We used foam weatherstripping on the door itself. That gap is gone.
On almost all of my windows I have room darkening roller blinds. I keep them closed in the winter and my lined drapes pulled closed. I figured that might help keep some heat in as well.
Hope this gives you a few ideas.