Feeling completely stuck on aspect—when do I use dokończony vs. niedokończony?

I’m an intermediate learner and I feel like I’ve hit a total plateau with Polish verb aspects. I understand the definitions of perfective and imperfective verbs, but I freeze up every time I have to choose one for a past-tense sentence. Are there any specific sentence patterns or 'flags' in Polish that signal which aspect to use, or is this something that only comes after years of exposure?