Add the following JavaScript to your wiki's MediaWiki:Common.js (or another JS file):
if (mwCustomEditButtons) { mwCustomEditButtons[mwCustomEditButtons.length] = { "imageFile": "http://upload.wikimedia.org/wikipedia/commons/3/35/Button_substitute.png", "speedTip": "Substitution", "tagOpen": "{{subst:", "tagClose": "}}", "sampleText": 'Template to be substituted'}; };
(For production use please change the imageFile URL to one on your own server.)