I know nothing about the sport but I do know a little about getting faster for rugby, hell, there might even be some crossover.
Listen to Shaf.
How far do you need to run? How fast do you need to run? How long do you get to rest in between?
That should set the parameters for your training.
I like the litvi stuff that Pierre mentions but keep it simple. Sled dragging, resistance bands, parachutes and all that shit aren't necessary. Get out there and sprint. 40 x 10 metre repeats. Focus on form (ask and I'll give you advice on running technique). Cranking up with the gimics can be very detrimental if you haven't honed the technique in the first place.