This question focuses on two ways in which salt can be a negative thing. However, it does not look at the ways in which salt can enhance life. Perhaps Christians who want to be the “salt of the earth” should focus on these positive qualities of salt and not on the negative ones.
Of course, Christians can be suffocating and abrasive if they are too ostentatious with their religion. This is only my opinion, but many Christians who claim to be bearing witness to God are acting like the people that Jesus criticizes in Matthew 6. They are praying out in the street so that others will notice how righteous they are. This can be abrasive and can suffocate any desire to be Christian on the part of many people.
But salt has other uses. It can be sprinkled over ice to melt that ice. It can be put in food in small quantities to make the food delicious. Perhaps this is how Christians should see themselves. They should be the salt of the earth by helping other people and not making a big fuss about their faith. By doing so, they might melt the hearts of people who would not respond to haranguing. They should be the salt of the earth by injecting their faith in small doses, not by being overbearing. In other words, they might simply say things in passing (when appropriate in a conversation) about how faith has helped them in a given situation rather than trying to give sermons about their faith.
In short, Christians can be the salt of the earth in more understated and less pushy ways. This will help them avoid being abrasive and suffocating.