Quite a few of my possible responses have already been mentioned, so I have to come up with something new.
I would advise Lucifer (the Lightbringer) to bring more light into our existence, and thus enable us to emulate heaven here on Earth. Also I would advise him to be good, loving, caring, to come to help in times of need - in other words, what one would expect from a “loving” God. This way he could actually earn our gratitude, not just expect it for no reason whatsoever - as God does.
And there would be another twist to that. Obviously, God would never do what Lucifer does, so God would be “forced” to become different. Since Lucifer would be just, good, loving and caring, God would have to pick up the “slack” and become hateful, cruel, mean and vengeful (like he used to be in the Old Testament), thus further decreasing the herd of his worshippers.
Finally, I would advise Lucifer to make hell a wonderful place, where we really could continue our life, without illness, pain, suffering. We would not have to sing that boring “hallelujah” day in and day out, we would not have to play the harp (we could just drink good Harp beer…) and generally have a grand old time.
Not that Lucifer needs any advice, since - according to the Christians - he already won. If you doubt it, recall the words: “narrow is the path which leads to heaven, while the road to hell is wide and straight”. Of course God’s PR people will state it differently. They will say: “While God was able to capture the prestigious second place, the Devil had to be content with the next to the last position!”.