IPad – Remote volume control on iPad

iSSH has a button on the toolbar for exactly this purpose:

Tap that button, view that menu, enable the feature!

There are Bluetooth remotes that can control the media playback functions of an iPhone, iPad or iPod Touch—if you can do what you want with the remote on Apple's headphones, these devices should be able to perform the same function. Typically the functions available are play/pause, fast-forward/rewind and volume up/down.

If you're using the built-in music player or any app that responds to the iOS media controls (an easy way to check is by double clicking the home button, swipe left-to-right on the task bar and see if the play/pause button works for your app). Because it's based on Bluetooth, the range will be limited to approximately 30 feet.

The two such devices I've found are the Satechi Bluetooth Multi-Media Remote Control and the Targus Bluetooth iPad Media Remote (which can also work with an iPhone or iPod Touch). There may be other similar products out there however.