This is what I've done:
1. Installed orientation lock applet as freemangordon said (in my case, by installing CSSU);
2. Set lock to landscape only;
3. Uninstalled orientation lock applet.

That way I do not waste screen estate in the status bar or in the status menu. My phone is permanently locked in landscape, although some apps (most notably Phone) force it to portrait temporarily.

I believe orientation lock will work without CSSU but I'm not sure. I have a faint recollection of having successfully tried that but I have tried so many things that I may misremember.

Edit:
Once your phone is permanently locked in landscape, you may consider removing rtcom-messaging-ui-portrait. It is for reading SMS in portrait mode.