I assume that you are running android 2.2 or 2.3. In later android versions the ability to move apps to SD was removed, but your post implies that you have moved some already, so I guess you have an older version which supports this.
If so, the first thing you need to do is set up adb (Android Debug Bridge) on a computer. You'll find some instructions and links in
this thread (the second post has the simplest download and install instructions).
When you have that installed, turn on usb debugging in the phone settings and connect to the computer by usb. Use the command "adb devices" on the computer to make sure you have adb talking to the phone.
If that works, then try typing the following 2 commands:
adb shell
pm set-install-location 2
This sets the default install location to the sd card. To undo it you use the same command but set the install location to 0 (automatic).
If that command doesn't work, try the older version:
pm setInstallLocation 2
Note that not all apps will work when moved to SD - if you want to use a widget, for example, you'll need to move the corresponding app to the phone's internal storage.
