Shift your receiving position somewhat to the left -- on both the deuce side and the ad side. How much you need to shift will depend on the specific lefty server. On the deuce side some lefties my not be able to swing the ball out wide so you might be able to make a moderate shift to your left. OTOH some lefties can hit a topspin or twist kick serve out wide on that side.
On the ad side, you may need to shift somewhat more than you do on the deuce side. You might need at least one of your feet in the alley (or behind the alley). Be prepared to hit BH returns against a lefty server. Quite often I will go right at the returner or aim for the right side of the body. In many of these cases, the returner will try to run around the ball to hit a FH return only to find that the ball breaks into the body making the return quite awkward. I call this serve my "heat-seeking serve".
As a righty server, you can attempt to do the same thing against a lefty server -- try to jam the lefty with sidespin on their forehand side. You can also swing your serve out wide to the BH side of the left, particularly on the ad side.