Learn from the masters:
http://youtube.com/watch?v=sd5XJGgvdXQ
http://youtube.com/watch?v=QdqTKPCfgC0
http://youtube.com/watch?v=Etv5XkEokHw
Against lower to mid level righties, you'll have a definite edge. Against upper level fighters, you pretty much have to learn how to fight like a righty (or at least think like one) since every advantage you have over them, they also have over you. Bread and butter shots include:
Any sort of pump fake followed quickly by a wrap shot to their sword arm. If you can get really good at selling pumps and don't abuse them, you will learn to love this shot. It also has the advantage of utterly confounding n00bs. It is the money shot for begining lefties. It's not as useful against experienced fighters since they'll see it coming a mile away and should already be set up to counter it.
If you can fake their shield out of the way, a high cross to their left shoulder can be a great move to learn. It takes quite a bit of practice and experimentation to learn how to finesse their shield out of the way. This works especially well against lefties, just reverse the motions.
I've found slots and stabs much more difficult to use against a righty, probably because when you move in for the slot, they can easily back up and pull a wrap shot on your back.
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The wrap and cross to their shield arm are the two shots that consistently land me kills against low-mid level fighters and should become second nature to any lefty fighting long enough. To add to that, there are tons of "trick shots" that I've found success with. The effect of a trick shot comes primarily from the element of suprise, so while they're good to practice, they can quickly become predictable.
A quick 360 pivot to your right can work as an excellent fake and frequently opens their right side to you (since it appears that you're moving to their left). It also adds momentum to your swing and gets a very satisfying "thud!' when it connects. Just make sure your back isn't over exposed.
One of my personal faves is to pump high to give the illusion that you're going to swing low, then tap the ground with your weapon and spring up for a cross to their shield arm. When it works, the "*?" reaction is priceless.
And for one final dirty trick: If you're fighting against someone who doesn't nescesarily know you're a lefty, hide your weapon closely behind your shield with your arms crossed to give off the illusion that you're a righty. When you move in to their left for what seems to be a wrap, pull your weapon out and instead wrap to their right side.
Remember that there is no actual advantage to fighting left handed at a higher level of competion. The best advice I could offer is to learn how to think like a righty. Once you know what's going on in your opponents mind, you will be better informed about which fakes and shots they won't be expecting.
Disclaimer: This is coming from a lefty fighter with several years of experience. I am merely average but am constantly learning and teaching. Take it or leave it.