Just a quick question, where do you stand?
Same spot as a righty serve, or what?
Not the same spot, and there is more to consider than just if they are right or left-handed. Your reutn position should not remain constant between different opponents, or even between different circumstances with the same opponent during a match.
I stand further to the left to return serve against a lefty vs a righty. Also, pay attention to their toss and their racquet motion. A lefty with a good slice can really hit the ball out wide. When you see that coming, take a step in and try to intercept the ball before it gets past the doubles sideline.
I agree with all of the above, but have a few things to add.
If you're playing a lefty who is "typical", he's going to swing you out wide on the ad-court (or down the middle on the duece court). He's going to hit slice serves wide to your backhand and he's probably going to do this repeatedly. He's probably found great success doing this with a nice feeling of comfort. There is more to it than making a position adjustment to help you return his serves.
You need to take the serve away from him. It's about taking action, as opposed to simply reacting.
Cheat over, cheat waaaay over. Take his favorite serve away. Force him out of his comfort zone and make him think about it. If he continues to try for better serives out-wide, you're in position to handle them, and he's going to start missing. If he tries to go down the middle, a much less comfortable serve for him, he's going to hit much less effective serves and hit them less consistently. If he makes them, you've still got a forehand to hit.
Both cases give you the advantage and over the course of a match the pressure adds up.
This doesn't just apply to lefty's and their serves. This applies to anyone with a favorite stroke which is fairly predictable. Take it away!