This has been a request for a while. It's not possible to reorder items in the sidebar. You can hide the entire sidebar and you can collapse the main items, but not reorder the items.

Here's a request to vote on:
https://www.rememberthemilk.com/forums/ideas/21471/

Thanks geojono. Hm, I see you asked for it 5 years ago and there haven't been many takers... I am really surprised it doesn't have more support. It's pretty crowded over there and it's really undermining my attempt to use RTM to replace Evernote as my main GTD tool.
But i see you haven't given up on it so I'll persevere too!

The developers are really good and I'm sure they're hard at work developing various improvements.

One thing that you can do in the meantime, however, is to mark items as Favorites. When you rest on a list, smart list, contact, tag, or location, you can click the little dropdown arrow and select Add to Favorites. That will create a Favorites section just below the top All Tasks section. That might be useful to you.

Hi colinlusk,
Thanks for your feedback! As geojono mentioned it's not possible to reorder the sections in the left navigation bar but by using the Favorites section you can approximate something a bit more custom: adding tags or Smart Lists, hiding other sections, etc.

I hope this helps a bit for now!