If you use Windows, do the following to foldit.bat. If you use Linux or any of the other Unix clients, do the following to the foldit script:
Open it up in a text editor. Notepad would work on Windows (Word would most likely not).
On the last line, there will be a "./foldtrajlite -f protein -q native -{other options}" (on Unix; on Windows it'll look something like ".\foldtrajlite -f protein ....").
Add -rt to the end of that line, save, and exit. Then restart the client.
If you want to reconfigure everything about the way the client runs, stop it, remove handle.txt from the DF directory, and restart it. You should be asked for all your settings over again when you run foldit (or foldit.bat) next.
This all applies to the command line client. If you're using the screensaver, it's probably an option in the screensaver setup (look for something like "use more RAM").