I understand it's the aftertaste which is stronger than the taste itself. Bitterness is tasted most powerfully at the back of the tongue, practically down your throat, so covering up the accessible part of the tongue with something nice, or lidocaine, is ineffective for this reason. Bitterness is also a slow burner, is not tasted immediately but within a few seconds.
Dignitas recommend their patients down their dose in two gulps. This is probably so the foul stuff is quickly down and the chocolate is in before the aftertaste truly bites with intensity. We drink 4 times the quantity, so probably worth practising drinking the equivalent volume in as few gulps as possible (say 8, for example) as quickly as you can. That way, come the day, you know what you're up against and will hopefully be able to quell the aftertaste in time.