Me and my gf recently moved in with each other. I was living with 3 other guys before. I've tried to explain about the toilet seat issue and that it may take me a little bit to get used to the idea of putting it down, but she really gets on me. It's not like I'm not trying.
And it better be a steep learning curve. The best advice to help you out is to post a note until you get in the habit of putting it down. It may not seem like much, but trust me, you don't want to encounter a female who sat down in the middle of the night only to find no toilet seat and plunged down into the toilet.