I hardly get any at all (though I actually don't do anything to discourage any spiders that have moved in), with windows open pretty much the whole time.
Maybe 1 'house fly' type a year, and the odd little thunderfly type thing taking a liking to my monitors at night.
Whereas in summer, house flies were often sneaking into my gran's house and zipping around the living room in pairs during the day, despite insect killer devices / fly paper, and her place was kept pretty spotless. No pets at either place or anything, just different locations.
Only thing that might be keeping them away from my place specifically is that just around the corner of the building are communal waste bins (enough to hold about 10 black sacks of rubbish each), perhaps they're just more attracted to that..?