webdiplomacy.net

Will my reliability improve over time if I constantly do turns properly and don't miss any? How will it improve, or will it?

No. Only the passage of time reduces the impact on your Reliability Rating from un-excused missed turns.
It doesn't help to do more turns that are on time as regards that rating, although people could look at your Game Stats to see how many games you've finished, and how many you've Civil disordered in, and draw their own conclusions.
For a full explanation, look at your WebDip Profile, scroll down to Reliability Rating, click on it, and then click on What is Reliability that shows up.

My understanding is that playing more turns reliably improves your "base rating" but doesn't affect the unexcused penalty. So if you have excused missed turns, they will hurt you less as you play more games. However, it's hard to get a really bad reliability rating just from the base score.

Ah yes, I forgot about the base rating component. Thanks for pointing that out.

Thanks. I'm not worried about new CDs, because now that I'm at school and have 2 hours set aside for homework there's a very small chance of me missing any turns, but all the CDs I accumulated over the summer are dragging down my reliability and I can't currently join many games unless I make them.

Some ratings seemed to be a bit outdated so I just forced a recalculation for all users. In your case, Baron, this raised your RR of about 6% since a "recent" miss (-11%) became "non-recent" (-5%).
However you have now 9 unexcused misses in the last 365 days: please be aware that just another one will result in a very long automatic ban!