How do I temporarily disable applications from starting up at login? Is there any way to temporarily disable applications from starting up at login? I'm referring to both Lion apps that are started due to the resume/reopen feature, and also to applications set to launch in the Login Items setting.

I know that I can disable the apps from launching in Lion by unchecking the relevant checkbox right before I shut down, but what if I forget to do this, or if I decide I don't want them to start up right before I login?

To clarify: I'm looking for a one-time, temporary solution rather than something that will disable them all the time.
Hold the shift key as the user log in process starts.

You need to hop on the shift key quickly if you enter a password right after pressing return, get on the shift.

If your account logs in automatically, look to get on the shift when you see the gray screen with the dark grey apple logo. Next comes the blue screen and then the graphical user log in portion. If you hold the shift too early (before the Apple logo) you will have a safe boot(not what you want) 

A safe boot has much more temporarily disabled than just the user log in items.

See the Apple knowledge base article for more information about items that open automatically at start up and how the shift key trick works.