My experience in sales
Last year, I worked as a server at a Vietnamese restaurant in an airport.
The restaurant faced a problem:  Sales to airport workers, which accounted for 20 percent of total revenue, was down [or: had fallen].
We had to address this problem because sales to regular workers were important.
In an attempt to resolve the issue, I tried to find out [or:  figure out] what could distinguish us from other restaurants.
I found that other restaurants were giving out two coupons to the workers, while we were only giving out one coupon.
As a result, I suggested a promotion that could be popular:  Customers could get three coupons once a week at peak hours (like lunch time).
It turned out that we could attract more regulars, and sales revenue went up by 20%

sales experience

My experience of sales
Last year, I worked at a Vietnamese restaurant in an airport as a server.
A main issue was rose  that sales revenue from airport workers employees went down which was previously accounted for 20% of the total revenue. And we have to deal with that because benefits revenue from (or "benefits for", depending on your meaning) regular workers were important.
In order to solve the issue, I tried to find out what would distinguish us from other restaurants.
After that, I found that other restaurants gave out 2 coupons for the workers, but we did only 1.
Then, I suggested an event that would get a increase our popularity, which was to give out 3 coupons in peak hours(like lunch time) once a week.
It turned out that we could attract more regulars and sales revenue went up by 20%

It was well written, so I have made changes to make the note easier to understand. Your original sentences may not necessarily be wrong. Good job and good job in improving the sales revenue. :)