Winters in old PNW houses is the worst. :) . Shrink wrapping the windows works, but don't forget about all the doors as well. make sure all the seals are good and tight, especially the one at the bottom. Everyone forgets about that one. if you have a crawlspace shrink wrap the opening to that too.
Finally, here is the biggest thing. Try not to use those wall heaters. They cost a fortune. Close the door to whatever room you're in and rock one of these babies. https://www.amazon.com/DeLonghi-EW7707CB-ComforTemp-Portable-Oil-Filled/dp/B002PLQ4T8/ref=pd_lpo_vtph_201_bs_t_1?_encoding=UTF8&psc=1&refRID=6RQB91WZBNN5NZ21SRNB
If you have access to the crawlspace you can throw insulation in there, but that's pretty expensive.
Get an oil filled space heater. Acts as a heat sink and is significantly less load over time versus a standard space heater. Running one on 750w should be relatively easy for almost any breaker to handle, unless the circuit in your room is ridiculously overloaded. If you can get away with the 1500 watt mode, even better.
Keep in mind it'll take an hour or so to heat up but you can set the good ones on a timer so they kick on before bed (or whenever) and shut off at a certain time as well.
I own two of these and love them: http://www.amazon.com/gp/product/B002PLQ4T8/ref=pd_lpo_sbs_dp_ss_3?pf_rd_p=1944687402&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000FT1XZW&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0ZC5CHX758ZHTC442P7V
This is the device that I use to control the heat in my room. It works really well and doesn't cost a lot to run.